When diving with limited visibility or at night, it's possible to get separated from your buddy. You're chasing down a lobster in a little game of tag while your buddy is following an unusual black stingray. You turn around a minute later and he's gone. Maybe you're fishing and just speared a world-record size halibut and need a hand getting it on the stringer. In any case, you've got to get your buddy's attention.
Some people actually try to yell into their regulator.
Now, if it's a nice, relaxed dive, you might hear that, but if you've got a hood on or are in any sort of excited state you might not hear that. They recommend using the tank banger style device. I decided to create my own device out of some spare crap. I put the handle in a vise, and using a pair of vise-grips, I twisted the driver shaft until it came out of the handle. I then drilled through the rest of the handle.