I think I though of a decent use case:
Canister A hods some state that is important to a decision on Canister B. Canister B could query Canister A for that state during an update query, but that will result in the cycle cost of a XCanister Call and a round trip for waiting for consensus.
Instead it would be nice if A could push the certified data to B once and then the client could deliver the certificate with the update call with a witness and validate any number of pieces of data on A without the XCanister call.
Example: A is a ledger of Tokens X. B awards a Token Y if you burn tokens X. A can update B on the root and all burns → mint can be executed without a XCanister call.