Yes, so that we can close them. Closure doesn't always result in deletion, but it will mark such questions as unwelcome and reset the precedent (the way questions are locked and retained on SO because of "historical importance"). Such locking would be better, but we'd need a mod on hand for that, I think.
Apparently "historical importance" is the wrong type of lock here. It freezes both question and answers, whereas only the question need be frozen, as Seth suggests. With the other locks, the existing answers can continue to be updated and new answers cannot be created.