I can't remember which video it is, but you should be able to see some videos here of Matt Cutts(from google) talking about this:
https://youtube.com/results?searc....1.AKHcsWFJyRE.
Basically when you link to site, its almost like giving it a vote, or saying I have confidence in this site. The more google likes your site, the more this 'vote' will count for. If you use no follow, your saying you don't want to pass any credit to this site.
Persoanlly I have never used nofollow as I have never had a reason to.