So many! The first obvious use-case is federated, encrypted instant messaging. The only two alternatives are delta.chat and Matrix. Former is pretty cool (everyone has email right?) but lacks good desktop clients, latter is pretty cool (decentralized rooms, "spaces") but server-side recommended software is resource-hungry (but conduit.rs is an ongoing alternative) and client-wise there's only one client that implements all features and it's a web client which is really bad for security and reasonable resource usage (here again alternatives are coming like Nheko).
The Jabber/XMPP ecosystem is more or less advanced depending on what features are of interest to you: PubSub-based microblogging has been standard for years, audio/video used to be a disaster but the situation has greatly improved with interop between major clients... while IRC-like IM stuff has been working fine and stable for almost two decades, a timelapse during most people had time to change addresses at least three times outside of that ecosystem (IRC -> MSN/AIM -> GChat/FBChat -> Whatsapp/Signal/Telegram -> Matrix/Briar/?).
So what's relevant about modern XMPP specifically? Account portability is being worked on as part of the Snikket project, and ActivityPub interop as part of the Libervia project. The former is a complete client/server XMPP distribution for easy selfhosting (no hard fork, only friendly forks contributing upstream), the latter is a library for building native XMPP clients (frontends) more easily which pioneered federated forging (to replace Github & cie) and is currently using itself for its own development.
So not everything is perfect and shiny but XMPP ecosystem is doing good after all those years. It's pretty easy to selfhost a server and get started. I just really hope we have more cross-protocol cooperation to promote interop between the different federated networks. It's a shame that some federated ecosystems don't value interoperability as a key feature.