The use case is privacy. Data getting harvested by free and even paid for services isn't pleasant (targeted ads, data breach etc)

If I get to add some "server" capability it will rather be webrtc, basically P2P to sync between devices, or a config to plug our own store. E.g GitHub, Google drive, dropbox or a self hosted service to SCP the files.

It isn't just browser cached, one can export individual documents or the entire store as a zipped folder. And back that up.