Along with allowing individual developers conversant in Ajax to bring Facebook friends into their website's user experience, as John Potter points out, it opens up a role for third-party developers to craft Facebook-friendly widgets that are easily dropped into blogs and sites that don't have any Facebook programming experience. The release of this library caused a fair amount of buzz over the weekend from folks including Nick O'Neill, Dare Obasanjo, Jeremiah Owyang, Duncan Riley, and Search Engine Watch.
Recent moves in data portability and OpenSocial-style compatibility suggest that we are moving towards an environment that allows some form of opt-in sharing between elements of the social graph, and Facebook wants to make sure that it is easier to identify your groups of friends by starting with their version. The function of allowing you to organize your friends into groups (family, close friends, business acquaintances, etc.) that was added in December is also a step towards making your control over your social graph easier, and adding lock-in to the Facebook data.