I talked to some people who did this some time ago because my friends and I had to write a paper for school. They have low self esteem and try to find a way to feel better. They use other people's pictures and even if they know it's not them, when they get compliments, they believe them. It does help their ego, even if it might not make sense for others, but the person they're creating kind of becomes a part of them so yup, the compliments they get have an effect on them.
Attention... it's all... it creeps me out how people go from profile to profile for hours looking at peoples photos... thus why i don't have one up lol you'd think they could do something better, i mean these days a lot of younger people are without jobs but you can watch DIY or cool life hacks or go on a hunting spree for a new programme or something lol that's what i do.
Attention and fishing for compliments, but honestly who really cares, they still aren't that person in real life and we know that so if it makes them feel good about themselves for a bit let 'em do it. I'd rather rate someone's fake pic and make them feel good about themselves even if only over the computer, then allow them to feel so bad about themselves that they end up doing something stupid... just saying.