I think it would be useful for those who have smaller keyboards or use special keypads for games and the like if the we were able to map the function key to a button on the mouse.

Example would be for a game that uses the number keys (0-9) and the respective function keys. Instead of needing both hands to press the function key on their keyboard to get access to the F1-F10 keys they could keep their hand on their mouse and press the Function Key there.