Matt Cutts explains: http://www.mattcutts.com/blog/goo-gl-url-shortener/

Q: Why are you doing this?

A: Google needed a url shortener for its own products where we knew the shortener wouldn’t go away. We also wanted a shortener that we knew would do things the right way (e.g. 301/permanent redirects), and that would be fast, stable, and secure.