It solves roster/presence and federation. The core of XMPP is pretty small; the extensions are numerous, but you don't have to implement them (if you're just us

It solves roster/presence and federation. The core of XMPP is pretty small; the extensions are numerous, but you don't have to implement them (if you're just using XMPP for the federation or as a messaging layer) for the most part. You also get the advantage of tried and true libraries for a lot of languages, which saves you from reinventing the wheel again. There are stable servers and usable clients to help you in development, and the documentation of the protocol is pretty good. Also using things like HTTP-Bind interfaces, you can basically use XMPP without keeping a connection open. All this flexibility has its use.