I'm no expert, but i think that it is so you can snug the butt up to your shoulder with your non-shooting hand if you don't have a back rest of some sort. I may be wrong though.
to grip with support hand, or to lock into a rear bag, or to push and lock into your shoulder with support hand when using a bipod or other front rest.
plus it hooks just right into the jawbone when you use it as a blunt object and looks pretty cool.
i find a "rabbit ear" style that have the two wings on top that look like, well, rabbit ears cradle and support the stock to deter front to back and side to side shifting
<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: StrikeHold</div><div class="ubbcode-body">You put your weeeeeed in it. </div></div>