There's a few use-cases:
1. Scheduled Tasks don't have a good data store to back them up. E.g. I needed a way to re-send webhook requests that failed because the other server was offline. Due to the limitations of Cloud Script, there isn't a good way to queue up these requests.
2. Can reify game concepts to entities. Currently, I have to use hacky string-based methods.
3. Just independent persistent storage.
Please post your own use-cases if any come to mind.