Continuity mistake: While up in the tripod's "holding cell" for humans, Ray spits the circular grenade pins into his hand, and their positions on his palm differ between the two consecutive close-ups. (01:40:15)
Continuity mistake: When Ray runs out of the house and meets up with Robbie, the cars and pedestrians very noticeably differ between consecutive shots while they talk. (Not due to cars being pushed.) (00:18:15)
Continuity mistake: Once Ray takes the piece of pita with hummus and eats some of it, the rest of the pita and the way he holds it differs in following shots. (00:12:30)
Continuity mistake: As the News Producer speaks with Ray amidst the plane wreckage, her black bag, her actions, and how she hold things such as food trays, change abruptly in consecutive shots, while Max searches for food in the locker. (00:43:15)
Continuity mistake: As the hole begins to crack in the overhead wideshot the crowd moves back, and the florist is at the top center of the screen with hardly anyone near it. Yet, in the next close-up, suddenly there are dozens of people standing in its path, as the crack dramatically approaches the florist. (00:21:15)
Continuity mistake: When the asphalt quakes in front of the church in the overhead shot, just as the huge gush of water shoots up from the ground, in the foreground a person is leaning way out their window aghast at the goings on. In the next shot that person is gone and the window is shut. (00:22:45)
Continuity mistake: As Ray says, "Next time you take my car with no driver's licenseĀ " he walks a good distance away from Robbie. Yet in the next close-up of Robbie, Ray's arm is still pointing up near Robbie's face. (00:18:25)
Continuity mistake: When Ray heads upstairs from Mary Ann's wrecked basement, there is a mess beside the stairs. However, in the next shot looking down, there's no mess near the stairs, furniture differs, and now there's a wicker hamper that appears out of nowhere. (00:41:35)
Continuity mistake: As the asphalt begins to burst open, the Portuguese restaurant's windows shatter, and when the camera pans to the church there are very few people on the street in front of it. In the next shot, however, there is a huge crowd in front of it. (00:22:10)
Continuity mistake: In Boston, when Mary Ann lifts Rachel, she holds her daughter then turns to face Ray and says, "Thank you." Behind them, up the stairs, the left front door is already wide open, yet in the next shot both doors are closed until her parents dramatically open them. (01:47:30)
Continuity mistake: When Ray is at the News Producer's van, she grips and starts to push the videotape with her palm up, but in the next close-up it's the opposite, she's pushing it with her palm down. (00:44:00)
Continuity mistake: Ray and Rachel are about to walk down into the basement at Harlan's house, the door on the right of the bulkhead is open and the door on the left is closed. In the view from the basement you will notice that the door on the left is open and the door on the right is closed.
Continuity mistake: Whilst Ray is trapped in the upside down pickup, he looks through the windshield hole directly at Rachel, who stands on higher ground. In the next shot from behind Rachel, the landscape, wood fence, etc., to her sides are entirely different from the previous shot. (01:36:35)
Continuity mistake: When Ray and others are crouched beside the hole in the asphalt, the guys around him and the way he holds the freezing piece of asphalt, differ between consecutive shots. (00:20:50)
Continuity mistake: When Ray reaches the red church at the beginning of the film, the sun changes positions three or four times behind the tower of the church. (00:20:10)
Continuity mistake: As Mary Ann's house shakes during the alien attack, when Ray shouts, "Where do we go? Robbie, this is your house. Where do we go?" Robbie is hugging Rachel. In this shot Rachel's hair is extremely curly, whereas in previous and following shots it's barely even wavy. (Most likely she's the body double.) (00:40:15)
Continuity mistake: In the countryside, after Rachel witnessed many dead bodies float down the river, and they head back to the van, an Army Convoy rolls by. While rolling by, Robbie starts jumping up and down waving his arms. His pants creep down enough to see his butt crack and reveal his red underwear. The next moment, picking up at a different angle, his backside is now covered by his grey underwear. (00:49:25)
Continuity mistake: As the boy runs up the stairs to the second level of the ferry Tom Cruise yells, "Robbie". If look closely there's a large speaker in front of him that can be seen in the first two shots but then disappears for the rest. (01:01:50)
Continuity mistake: When Rachel says Ray should get TiVo, she is looking at her hand. When it cuts she is suddenly resting her head on her hand. (00:11:55)
Continuity mistake: When Ray runs behind his house to have a better view of the sky we can see from behind that his hood is blowing in the wind. When it cuts to a shot in front of him his hood is still. (00:13:40)
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.