You can actually drag a file from one window to the other without having both showing (such as between two maximized windows). While dragging a file, you can still manipulate windows around using the keyboard, like with alt-tab.

For example, you can begin dragging a file, then alt-tab to another window or tab or even open a new program or whatever using the keyboard, and then finally release the file into the destination window.

It's not obvious that this would work, but it is convenient if you find yourself wanting to drag something with the mouse for some reason.