This is useful for any collection that is also a capped collection (that is, the collection has only a certain amount of finite space allocated to it), not just the oplog. In fact, the oplog is just a special capped collection, hence why tailable cursors are available for it.

Also note that non-capped collections don't support tailable cursors, so this won't work. Documentation: https://docs.mongodb.org/manual/tutorial/create-tailable-cur...

*Edit: clarified that the oplog is a capped collection for context