I like being able to hang cordless power tools from a belt, particularly when climbing ladders. I have a hook on my tool belt, but it's far more versatile if the tool itself has the hook.
I'm still refining the design to allow for customization, but the current test parts fit my Ryobi drills and drivers perfectly.
Update 2017-02-23: various tweaking, now customizable, and when I saw this it occurred to me that I could fit a sweet little bit holder into the clip...
Update 2017-11-04: after my driver fell from the rafters a few times, I broke a clip. New version beefs up the place it snapped off.
Put the loop around the handle as low as possible, insert the clip part pointing towards the battery and secure with a #8-32 machine bolt and nut. It's likely the holes will need to be reamed out with a drill bit.