The only reasons to do free work.
- Because you want to. Part of the fun of being an artist is making art for the hell of it. Sure being a "starving artist" is no fun, but being a slave to getting paid isn't that much better. If a project catches your interest and you would like to work on it, then by all means go ahead and have fun!
- Experience. I become a better Filmmaker after every shoot. In fact, the more I film the more I realize I still have a lot to learn. The thing about Film, and most art forms, is that you learn better by doing, so if experience is what you're after, there's no better way to get it than by practicing. Gaining experience also puts you in a better position to charge later on.