What's the little plastic clip on the headset good for? On my iPhone headset (the one that came with the device, as well as the in-ear buds) there's a little piece of plastic that slides on the cable to one earbud, and can clip onto the other cable. 


It's not large enough to accommodate the headset cable below the split, so it can't be used to somehow hold the cable in place when it's curled up. 
It's not tight enough to stay in place if I slide it nearer the earbud, so it can't be used to essentially move the point where the cable splits closer to the earbuds.
It's not tight enough to really hold on to the other earbud cable. It takes nearly no effort at all to pull the cables apart.


What in the world is it good for?!?
I actually find it very useful to clip around the buttons on my dress shirt. Depending on the amount of slack I want, I can pin it around different buttons.

It creates enough slack in the lines/wires that go to the ears:


reduction in cord noise carrying up into ear-buds
the mic in the right wire stays away from your body (better pickup and less noise
if the earbuds fall out of your ear, they don’t drop all the way to the floor 


It works perfectly (see yellow arrow in picture):