I do know that if you attempted this right now, all of the instances would need to access the same database somewhere which would retain a bottleneck.
Switching to a different distributed (eventually-consistent) database may be possible for Lemmy as a software but perhaps not possible (or difficult) for ActivityPub. I don’t know that.
I don’t have it myself but I hear that Jerboa requires the instance you signed up on to be (lemmy) version 0.18 as of a recent version.
Useful for grabbing and keeping a personal backup of emails. Can be referenced offline if connection to the internet is poor. Native notifications. Better integration into other desktop applications. For Apple users, good syncing across devices.
Obviously web clients could also provide most of these benefits as well.
…and the things which aren’t are foundational elements of the republican party’s platform.