If you are dragging an element onto a window which is hidden under another window, the target should pop up on top, so that you can verify it's the correct one, or choose the correct target element of the window 

e.g.: you're dragging a file onto a nautilus window, where you have the left panel open and the file area is hidden. You drag the file over the window, and after a while the window is bringed over, so you can release the file onto the desired directory/position.


Other information: