Maybe it is because I've only ever worked on web applications so I can't imagine the use cases, but when would you ever want your database to be replicated to a client's computer?
One of the use cases for RxDb is "local first" data. Rather than spend time doing roundtrips to a remote server for each piece of data, store the database locally on the client and keep the local/remote in sync.
IndexedDB for the client, web workers to check that things are in sync and events for when things aren’t.. best of both worlds