An use case i find interesting is to cut steps on contracts and swaps.
If a contract uses HTLCs and end up with a privkey handover, then the other party already have access to the musig, however, today we need a final tx in order to be certain that no HTLC will be published onchain in the future. With HTLC expiration, we can make swaps 1 tx long, something that can be useful in a high fee environment.