The client library allows users to make Facebook API calls from any web site and create Ajax Facebook applications on that website.
Wei Zhu from Facebook explains the benefits:
Since the library does not require any server-side code on your server, you can now create a Facebook application that can be hosted on any web site that serves static HTML. An application that uses this client library should be registered as an iframe type. This applies to either iframe Facebook apps that users access through the Facebook web site or apps that users access directly on the app’s own web sites. Almost all Facebook APIs are supported.
Nick O’Neill at All Facebook writes:
Want to build your own social gaming platform that resides on your own website but leverages the power of users’ Facebook relationships? Now you can! There had previously been applications that could leverage the Facebook API prior to the launch of the platform but there are some significant differences now versus before. The first significant difference is the broader access to Facebook’s core features that the platform provides.
I’m not sure anyone saw this move coming, but Facebook may have just changed the game again by essentially becoming an application host. It’s a clever move by Facebook in a year its competitors will get more serious about offering platforms themselves.