Because it does someone no good if you have a copy of an old page they're looking for, and they have no way to find you, and you have no way of providing it to

Because it does someone no good if you have a copy of an old page they're looking for, and they have no way to find you, and you have no way of providing it to them. Centralized services like libraries and archives provide that. You absolutely should save everything yourself. But you should also give copies to as many centralized services as possible. Lots of copies keeps stuff safe.