When navigating a tree structure.
Processing files and folders, expression parsing, that kind of thing.
I've no idea why the factorial example is so popular, because it's one of the worst use cases for it. Still, I guess it can teach a new programmer what a stack overflow is.
Real decentralisation would mean that the accounts and communities could have multiple homes (added by the account or community mods so private data doesn't go anywhere you don't trust), so people could just carry on when a server admin forgets about it.
We're not there yet, and I don't think the current Fediverse really handles it. Something to work towards, I suppose.