>Mitra exists as a practical implementation compatible with Mastodon, and it implements...
The main specification is FEP-ef61, which describes how decentralized ("nomadic") identity works. FEP-c390 is an older idea and it doesn't provide full data portability. FEP-ae97 is about combining FEP-ef61 with ActivityPub Client-to-server API.
>I just don't fully understand what that gets us.
You can clone your account to multiple servers, and those multiple accounts would work as one! And more: peer to peer, local-first applications.
Mitra's implementation is actually incomplete, I recommend also checking out Streams, which supports FEP-ef61 and non-ActivityPub nomadic identity (Nomad protocol; this is where the "nomadic identity" comes from).