Allow people to use their own data...
... empower people–everybody loves talking about stuff
that empowers them ...
... which spreads the word about your service
Developer community–the ultimate power users
Extend your application–solve niche use cases
Conceptual clarity–the essence of your service
Inspired by http://strange.corante.com/2006/02/08/fowa-ten-reasons-why-you-need-to-build-an-api-shaun-inman