Why do people create fake profiles?

So it's happened time and time again on every forum app that I've used. People create fake profiles using pictures they find on google and make up stories about their lives. 
I always wondered why people do it and what they get out of it. Any thoughts, ladies?  Do you think they try to recreate themselves? Do you think it is possible to stop them from doing it? Do you think they should be stopped?  What should the penalty be?