Can I do this? Am I skilled enough to handle that task? Its honestly what starts most of my silly little "stupid" side projects. I want to see if I can handle something. I want to see if I am up to the task.
I find the side projects where I focus on something to be just see if its possible. And usually its a copy of something that exists to a lesser extent. Its ideas that I see in movies, and tv. Terrible ideas that should never be brought to life but doing so might be a fun little thing that I can do on the side. Other times is an idea someone like 
Usually these types of things just end up getting throw in my folder of other random garbage, probably never to see the light of day. Most people probably won't even know they exist.
But one day, I usually end up needing to do something which I can borrow code from those projects. Thats why I love doing them. Have fun now and have work done for the future. I can probably spend a lot of time just starting new projects, its the putting them all together part that gets boring. When ideas are fresh and awesome in my mind, they get interesting to do.
So why do I make some of the things I do? Because I want to see if I can. And if I can't? Well I make sure that I can.