Because I'm a programmer
Marco Arment, in the README for his Second Crack project, answering "So why did you make this?":
Because I'm a programmer, and this is what I do.
Some people jog away from their house every day, only to jog back. Others walk on a treadmill, expending energy to get nowhere. In both cases, it may appear to others that they've accomplished nothing, but they've chosen to do these seemingly redundant activities on a regular basis to incrementally improve themselves. And it works.
Inspirational. Also, I am glad to finally be able to peek at the Second Crack source.