What are some applications of this? If an attacker has access to memory there's not much left you can do right?

Or is the use case similar to why you would want to zero out the memory used to store password strings?

There are lots of scenarios where you would want to let an untrusted intermediary handle critical operations for you without actually having to build trust with them.

Typical example: Certificate authorities distribute CRLs primarily through untrusted channels and third parties to lower their operating cost. The CRLs are signed so its a win-win for everybody.