I do them because it allows me to have complete creative control. I can run with ideas that are unique and fun but a client would probably never go for. It also means you can fill your portfolio with work that you'd want to be hired to do.
Personal projects are a great idea. For me The Print Handbook started as a personal project while I was working at The Media Collective. Now it pays for some of my bills and is shipped all across the world.
That said, I don't think a personal project needs to have a commercial focus. I've done many things just because I wanted to. Personal projects allow you to do the bits you love most about design without having to track your time, bill clients, reply to emails, etc. You just mess around with no pressure.