What a repeater region does is let you pick an area of your screen that will automatically pass the mouse and modifier keys to a given window or windows.

The obvious use for this is to have healer addon unit frames in the same position on all windows, and a region over it to pass the mouse to the healer, as demonstrated in Svpernova09's links. This allows healing from any window, without making dozens of hotkeys -- previously you would have had a macro for each heal for each character you wanted to use them on. Some people actually currently have 25 macros set up just for healing. The rest of us prefer to heal the same way we would if we were playing the healer solo