Suppose someone makes a big change on page Foo or Template:Bar, and gets reverted because some other user found a problem in the new revision. The first user may want to improve its original change, and to do that, he/she may open his/her old version from the page history (i.e., oldid=12345) and then open that old revision for edit (oldid=12345&action=edit).
Once the updates are finished, though, there is no way to get a diff of what he/she changed since the last try. He/she will need to save the page first, and only then to get a diff.
So, I think it would be good to provide a new button [Show changes since this old revision] on the right side of the existing [Show changes], when editing an old version of a page.