Take just for example (if it's a feature, I don't know) setting yourself to an away state. In a client-server model you'd just tell the server you're away now with any metadata that goes along with it. Other clients will get that information from the server.

With a p2p model you have to tell every contact's client that you're away and provide any metadata that goes with that information to each individual contact.

To reiterate the away state is just an example off the top of my head, I don't know if this service supports status updates.