Handgrips are a way around ergonomics.
When the shooter is wearing body armour, there is no "natural" butt pocket. Soldiers are taught to square their plates to the threat and hold the rifle on the chest portion of the vest. Collapseable butts make part of the triangle manageable, but don't solve all the problems. Proper body armour has protection that covers most of the ribcase, and it is harder to raise your arms into a more comfortable position. Then the left hand can't go very far out on the foreend. Hence, 75 new products every SHOT Show to give the shooter something to hang onto.
FWIW, an internet superstar called Instructor Zero uses a front handgrip that looks like a paddle grip off a router. It is offset from the axis of the rifle, hanging off the equipment rail.
