We don't always notice them so we don't always delete them. Some don't seem harmful. If they get nasty, you can bet we'll notice.

Folks use a different identity because they're afraid they may suffer, even possibly when they're just stating an honest (if unpopular) opinion. Of course, a person may praise himself. It's maybe be a bit on the sad side or maybe just goofy. I don't know what motivates it. Maybe a person thinks he's putting one over on folks. If people found out, he'd probably lose people's respect, but it isn't really hurting anyone... other than the person doing it, I mean. Falsehoods take far too much effort to protect.

On the internet, they're called 'sock puppets'.