
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.
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.