You may've heard of OpenID - it's a distributed identity management system, a.k.a. a decentralized single sign-on platform.
The reason they do this is that signing up is actually a big barrier to entry for users of web apps. Users feel more comfortable when they don't have to sign up to use an app - it's much easier to give it a try and it's less time-consuming to start using it. That's why most web sites today try to keep sign up process as short as possible.
Although Google's approach seems more successful now, and the other bigcos have adopted the search-centric model, single sign-on is still a very important paradigm. Therefore OpenID can provide the advantages that Google, Yahoo and MSN have, to all other independent sites - in a decentralized, open fashion.
Acknowledgement with gratitude:Read write web