Using spaces for asset storage with the CDN overlay is extremely convenient. However, sometimes it is desirable to make a change to just 1 object that is cached, and to be able to invalidate that from the cache, rather than wait out the TTL, or blow away the entire cache which would be inefficient.
Once concrete use case is having a number of JSON files that act as manifest indexes for other objects within spaces. External web-apps use this to determine what to load. These change somewhat frequently, but none of the assets that they point to change. The client app just needs to be pulling the latest JSON (within a few seconds of change, not an hour).