Successful moving/copying operations can be undone (ctrl+z), so dolphin moves the files back to their source location (if you moved files)
or asks if you want delete the files (if you copied the files).
But the same is not valid when these operations are not completed (because free space available in destination is not enough to all files, for example). In this case, nothing happens or dolphin reverts a previous successful operation when I press ctrl+z.

Functional ctrl+z is useful for easy reverse not completed tasks specially when moved/copied files include files that depend on other files, like parts of a split rar/7zip file or files of a dvd_ts folder, for example.