Continuity mistake: On the ferry, Rachel wears the purple/pink camouflage hoodie over her orange knit sweater with the colorful stripe sleeves, as usual. Now this is where things become strange. In the first shot as the trio come ashore, she is only wearing her orange sweater, she is not wearing the purple/pink hoodie - note the clear lack of fur hood. In the very next shot as they climb a bit more and then rest, Rachel is wearing the purple/pink hoodie - note the hood, but now it's on under her orange sweater. In the underwater shots she is also wearing it under the orange sweater. (This odd mistake has absolutely nothing to do with the very long wool cloak that Rachel wears in the next scene). (01:02:20 - 01:04:40)
Other mistake: When Ray pulls up to Mary Ann's home, the front exterior layout and dimensions of the house are evident, from its near center front door to the two car garage, in front of Ray's van. The side exterior wall contains two large garage doors, which are about 35-40 ft from the location of the front door, with no small basement windows at ground level; inside, beside the front door, the stairs that lead to the basement run parallel just under the stairs in the foyer. In the basement, the small windows on the far end of the furnace room they run into are only about 20 ft from the basement stairs. Not only are the small windows non-existent in the exterior shot, but it's entirely impossible for two small windows to be where they are, considering the exterior footage of the house. (00:34:30 - 00:40:20)
Continuity mistake: When the ferry casts off in the wideshot, there are only six people hanging off the outer edge of the ramp as it's being raised. When Robbie and another gentleman help these people over, in the close-up one of them wears tan pants, a long blue coat and knit hat, but this person was not amongst the six in the previous shot. (01:01:55)
Continuity mistake: When Ray and the kids are hiding out in the mom's basement, they take cover in a small utility room that goes pitch black when Ray shuts the door - even though there are windows above the girl's head that emit light before he shuts the door and after he opens it to leave. (00:40:35)
Visible crew/equipment: In Mary Ann's kitchen, just as Ray comments, "Ok, well you hungry? I'll get you some food," the clear reflection of a male crewmember moving around is visible in the glass window behind Ray. Then just as he says, "This is ketchup and mustard," another older male crewmember is visible directly in front of Rachel's reflection. (00:35:45)
Continuity mistake: When the van is parked at the side of the road, as Rachel heads off Ray shouts, "Every time you guys don't listen to me I'm telling your mother..." In the next shot the van is seen quite a distance away as Rachel reaches the shoreline. After seeing the floating bodies Ray picks her up, turns around and as he begins to run the van is seen, only this time is much, much closer. Ray never moved the van, as shown in the following shots. (00:48:00)
Continuity mistake: In the exterior shot of Mary Ann's house, there are three large windows of the same size over the front door, and large bay widows on both sides of the door. In the interior shot, when Ray remarks, "They're never here," the window over the door is visible and it is one large center window with very small windows on each side. In the dining room there are two windows with wall in between, certainly not a large bay window. (Additionally, in the living room there is a fireplace, where there were two windows in the exterior shot - but some could say that those windows were boarded up and a fireplace was built in front of it.) (00:34:30)
Continuity mistake: In Mary Ann's basement, when Ray and Robbie are looking out the small window, as the aliens begin their attack, Rachel runs over and Robbie hugs her tightly. However, in the next shot of Robbie and Ray, Rachel is gone, yet in the very next close-up Robbie is still hugging Rachel. (The shot of Ray and Robbie without Rachel is cut from a previous shot, before she runs to them.) (00:39:30)
Visible crew/equipment: When Ray and Ogilvy play tug-of-war with the gun, as the aliens rummage around the cellar, Ogilvy finally pulls it away from Ray. In the next close-up of Ogilvy and his gun, the boom mic dips down for a moment, but it's gone in Ogilvy's next close-up. (01:28:15)
Continuity mistake: When Ray parks the van at the side of the road, the van is about 60-70 ft away from the tall wood post, at the road intersection up ahead. However, in the first shot of the military vehicles approaching, the van is parked directly in front of that wood post at the intersection, and this also occurs in the shot as Ray shouts, "Robbie! You wanna go in that direction?" (00:47:40 - 00:49:00)
Revealing mistake: When the Hudson Ferry captain casts off the large label that reads "Hudson Ferry" is on his left side of his jacket. However, in the exterior shot as he looks out the window at the whirlpool, the label is on his right side. This shot was flipped, and amusingly, the CGI guys actually retouched the words "Hudson Ferry" so it wouldn't read backwards. (01:02:25)
Continuity mistake: When they stop so the kids can go to the toilet, in a long shot of Ray trying to persuade Rachel not to go too far, we see him start to open the boot (trunk) of the car. When it cuts to a closeup he's not. (00:47:50)
Continuity mistake: As Ray heads to the 26 strikes spot, while speaking with the two guys, he passes many stalled cars, some of which change entirely between shots. For example: In the close-ups he passes a red car in front of the restaurant, with a grey double parked car, and a white car at the street corner in front of the red, yet in the overhead shot a red car is at the street corner, with a grey car behind, in front of the restaurant. (00:19:30)
Continuity mistake: When Ray runs to catch the ferry, Sheryl loses her grip of him and when the soldier abruptly stops her the green hat falls behind her. Yet as the ramp rises, when Ray shouts, "There's room on the boat," Sheryl's hat is back on her head. (01:01:05)
Visible crew/equipment: In the scene when Ray is fleeing Mary Ann's destroyed home in the van. Right before he says "Robbie, get in." You can see a cameraman in the reflection of the van, right before it stops.
Continuity mistake: As Ray speeds away when Manny's shop blows up, Robbie shouts, "Holy s**t!!" and ducks. The rearview mirror on the van's windshield, in this close-up, is entirely different to the one seen a few shots later, when they're on the freeway. The mirror changes yet again later. (00:31:30 - 00:32:15)
Continuity mistake: When Ray and Robbie are throwing the ball near the start, check the wall behind Robbie. In closer shots of him there's sunlight on the wall and the shadow of a nearby satellite dish is visible, but in wider shots the whole wall's in shadow. (00:09:00)
Continuity mistake: While in his bed Robbie watches TV, as he leans against the wall behind him. Yet, in the close-up of the TV when Ray clicks it off, Robbie is clearly reflected on the TV screen leaning near the window, which is a good few feet away from where he was in the previous shot. In the next shot he's back to where he was in the first shot. (00:08:30)
Audio problem: The audio of the "SpongeBob SquarePants" episode on the T.V. doesn't match with the episode. (00:11:20)
Continuity mistake: When Ray steals the van and speeds away there is a registration sticker at the corner of the windshield, on the driver's side. The sticker is gone when Ray starts shouting, "You saw! We're under attack!" when they're on the freeway. (00:30:10 - 00:32:15)
Chosen answer: In the original George Pal version they were Martians and the reasoning for what they were doing was never explained. In this version, it's never explained where they come from, but their mission is simple, to eradicate human life from Earth, and use our bodies to fertilise the planet, probably so that they can colonise the planet for themselves.
GalahadFairlight
If it was to eradicate us they could have done that millions of years back, why now, so that doesn't add up.
You want to grow the substance (people) that grows your food source before using it. If they waited too much longer, they'd have a harder time because we'd have the technology to fight them back.
The reason which was apparently provided by Wells was that Mars was dying by lack of natural resources and that Martians needed a new home and food source.
They were waiting until the population grew large enough to sustain terraforming efforts. As they used our bodily fluids seemingly as a primary material for their terraforming.
It's an assumption that they could have eradicated us millions of years ago (which by the way would be long before we even existed). Maybe they didn't have the ability to transport themselves, only the machines. Maybe the original aliens all died. Lots of other options why they couldn't have done it.
They probably needed to wait for us to produce enough humans to use as fertilizer. Doesn't make sense to try to use several million bodies as fertilizer back then vs now with billions of people.