Originally I joined mainly to experiment with simple webpage design with a view to maybe one day developing my own websites. Maybe also to eventually make money.
I soon found however, that the main appeal for me was simply the pleasure of writing, and hopefully giving some small pleasure to others when they read something I've written. It's really quite nice to get compliments for the first time in your life on work that you've created.
Just 4 days ago I wrote a hub on this very subject, trying to analyse the various reasons why people write on HubPages.
If you read it, hope you agree with the reasons I give for writing on this site.