I work on side projects (my own and others) for a number of different reasons. But primarily to see how I can solve problems. Working on non-time critical projects means you can get a bit more creative and explore a lot of different options to see which work the best rather than the quickest to integrate. Some of the side projects I work on add a little extra rev every month, some don't. The only side project that consumed a good deal of my time is the one that in fact doesn't make much money (I probably lose every month) but its the one with the largest user base and its the one that is more of a love rather than anything else.