Imagine you have added a Curves adjustment layer and have spent some time painting on the mask. Then you decide that you should have used a different adjustment layer. Rather than starting over again, just go to the Layer menu, choose Change Layer Content, and pick the new adjustment layer. The mask will be preserved.
This is also a great way to add more than one adjustment layer with the same mask: duplicate the adjustment layer and then change the content - that way both adjustment layers will have the same masked areas.