This is not about prints. The purpose of such gestures is first and foremost the storage of negatives—to ensure their security and longevity. Original prints are often also part of it, but it's the negs that are most important to preserve in optimal condition.
From these negs new prints can be made—and if the photographer was its own printer, notes are often included in the archives, so the new prints can be as faithful as possible to the photographer's intentions. These new prints, if the original is not available, can then be lent to other museums in case of touring exhibitions or special retrospectives. That a museum owns the negatives makes these kind of exchange between museums much easier.
And sometimes these new prints are indeed sold. Magnum recently did this with prints by Capa, Erwitt and others.