Hundreds of millions of Facebook and Google users will be able to easily sign in and interact on Yahoo! using their Facebook or Google IDs. This eliminates the proverbial necessity of registering for yet another new ID and remembering yet another password.

From Yahoo!'s perspective, any signed-in user engaging with Yahoo! services is a valued user, whether she authenticates using a Yahoo!, Facebook, or Google ID.
Source : http://developer.yahoo.com/blogs/ydn/posts/2011/01/third-party-auth/