It works without a connection
A realm is kept on your device as well as on the server. You can write in a basement with no signal, and it syncs when you surface. If two devices change the same thing, the app notices rather than silently overwriting one of them.
You can take it with you
A realm exports as plain, readable JSON, and there are working export bundles for virtual tabletops. Being able to leave is a feature, not an oversight.
Private means private
A realm is private by default. Nothing about it is public until you publish it, and the difficult, personal material that campaigns sometimes hold stays where you put it.