if you're talking about doing database queries on a 5mb database, why not just ship the database client side and have them do the computation?

You may wish to build a protocol where third parties can asynchronously operate on user data. You may also want to have separation between the end app and the compute layer for legal or practical purposes. Finally, you may not want to store large payloads on client devices.

5mb is hardly a "large payload"

I'm giving you general reasons why this is the case. For our own app, we hope to build a protocol where third parties can operate async on user data (with consent).