factory wheel offset?

Is there any real performance reason why the wheels are offset? cause if this is a daily driver like mine is it starts to suck cause you can't rotate your tires or anything.. just buy new ones. So i guess what i'm getting at is i'm thinking of switching my tire size to 235/40/18 on all 4 tires... would this have any real affect on how the car would handle or any other aspect of things?