As stated in the topic, what exactly is a chin slider on IEMs used for? I never really found it usefull

For me, it helps tuck in the wires from snagging, and if the wire is over-ear makes for a bit of a tighter fit so the wires don't slip off from over the ear. 

often it helps reduce the noise that travels through the cable caused by friction or banging against it.
 
it also can help keep the headphones secure to your head, especially if you are wearing them so that the cord goes behing your head
 
in general it can help the cord from becoming unwieldy
 
lastly, if you push it all the way up against the iems before you wind the cord up, it helps keep them less tangled.
 
i think that about covers it.

i always cut them off. just gets in the way for me and i feel like it increases potential for unnecessary stress and entanglement.

If you are at the gym, and put the cable behind ears and down your back and cinch the chin slider up a bit it keeps it completely out of the way.

And... it also helps relieve stress at the "Y", providing a little bit of extra snag protection for basic cables.