I do fun stuff for fun. Maybe write an StrongARM emulator just because. Maybe learn a new language. Get a MIDI keyboard to see that I really can't play. Study virology because viruses are cool. Code a FTP server because it's "easier" than to find one that actually works as you want. Or an NFS server to share a folder from Windows. Fun stuff.
I might sometimes also work, but that's beside the point.