This is great if you already opened a multi-column view to a specific folder and then open a new window to find an item you wish to drag to that location. Often the new window will obscure the target folder - but hovering over the title-bar or border of the background window will bring it to the forground and allow you to drop the item right where you want it.
[Editor's note: This is a feature of spring loaded folders, and they must be enabled (Finder -> Preferences) if you want the focus to change on hover. However, if you prefer to leave spring loaded folders disabled (as I do - too many accidental mis-filings!), then you can still use the 'force' method to change focus -- just hit the space bar while the dragged object is over some portion of the window you wish to focus on.]

