Sometimes it's not about wanting to make money, about wanting to sell, or even about wanting to be useful. It's about having an idea and wanting to make it work. And if fame or fortune come your way as a result, great. But the real motivation is building it and seeing it work. (And maybe watching people go, "Wow! That's so awesome/amazing/elegant/etc!" But as long as you get to build it and it works, that's what's really important.)