Continuity mistake: When Hal's dad is on his deathbed, his hospital gown is wrinkled, including a large fold across his stomach. There is then a shot of Hal with his dad in the foreground and the gown is perfectly smooth. The next shot shows the dad again and the wrinkles are back. (00:03:20)
Continuity mistake: In the scene where Hal and his best friend are having a chat at the bar, Hal's best friend's beer refills after every scene when he takes a drink of it. (00:06:50)
Continuity mistake: In the scene with Hal and Mauricio in the bar, they both have a beer. In the shot where Hal asks if things are okay between Mauricio and his girlfriend, Hal takes a drink from his beer and Mauricio is holding his beer down by his chest. In the next shot Hal is suddenly not drinking his beer anymore and it is out of sight and now Mauricio's beer is by his mouth. (00:06:55)
Continuity mistake: When Jack and Dave are talking about the promotion, in a shot behind Dave, Dave has his hand raised, but in a front shot his hand is down on the table. (00:09:55)
Continuity mistake: After the elevator stops, Tony puts his right hand on the top of Hal's arm but when the shot changes it is on Hal's shoulder. (00:12:50)
Continuity mistake: When Hal is sitting in the elevator with Tony after it stopped, he is talking about a "severance pop." In this shot, the collar is his shirt is sitting out over his black waistcoat. But in the next closeup, it is nicely tucked under the waistcoat. (00:13:15)
Continuity mistake: After Hal asks Katrina on a date, and he thinks she's messing him about, Katrina takes a pen from her bag. In one shot she holds the pen so the nib is upright with no lid on, yet in the next shot she is holding it the other way round, and the lid is on. (00:19:45)
Continuity mistake: When Rosemary and Hal are talking in the fast food diner Rosemary's chair breaks and she is looking at Hal on her side, but when the camera changes view she is laying on her stomach. (00:30:05)
Continuity mistake: After two men insult Rosemary, Hal goes back into the restaurant. In one shot the restaurant door is nearly closed behind him, shot change and the door is much wider open. (00:30:55)
Continuity mistake: This mistake happens when Rosemary and Hal make up for the first time. Rosemary is standing in the doorway and she is holding a tray containing two coffees. As the shots goes from Hal to Rosemary, you can clearly see the coffees switch sides on the tray. (00:44:30)
Continuity mistake: As they're leaving the house at the end, just before they drive off, they show the car from front. Behind them, you can see Jason Alexander with his hands in his pockets. Next shot is from behind the car and he has his hand up in the air waving. (01:40:50)
Continuity mistake: When Hal's co-workers bring him a cake for his promotion, Rosemary takes a slice out for herself. They change shots a couple times, and the size of the piece missing from the cake changes several times, and the layers of the cake change as well.
Continuity mistake: In the scene where Hal is confronting the two guys that made fun of Rosie he points to Rosie outside the restaurant and her shoes change from brown and black clogs (which is what she was wearing throughout the scene) to a plain black slide. Watch, it is totally obvious because they show her shoes several times before they suddenly change.
Continuity mistake: At the DoubleDoor Inn, where Rosie is seen dancing with the guy with spina bifida, we see the other guests applaud and approach them once they have finished dancing. And if you look closely you will see that Hal joins the crowd at the right hand side of your screen. But as it cuts to the next shot where Rosie is walking away from the crowd, we see Hal catch up with her from the opposite side where he was standing with the crowd.
Continuity mistake: After Rosemary cuts the cake, she lays the knife down and in the next few shots the knife changes position from laying down to sitting up back to laying down again.
Continuity mistake: In the first club scene, Jason Alexander is dancing with a swaying woman, behind them is a woman wearing a headband and sleeveless pink top; as the camera pans around the corner to Hal's new dance partner who is holding on to the railings, the same girl with a headband is dancing next to them, only this time with a different guy.
Continuity mistake: When Hal is visiting Rosemary's parents for the first time, right before they start to eat, it is dark outside with crickets chirping everywhere. But when they start eating, you can easily see sunlight through the windows and all over the dining room.
Continuity mistake: When Hal is talking to his best friend in the gym when his best friend talks about "cankles," the machine weights in the back change numbers from frame to frame.
Answer: I think he was in the movie to reinforce that she is a genuinely nice person and that she had a wide circle of friends who appreciated her.