When kids buy a backpack for school they often have no idea that adults use the same backpacks for a variety of activities and not just to haul books and binders around, so many of the bag's functions remain a mystery.
One of the most common types of backpacks used by kids is "the one with the diamond on it", and that little leather diamond is doodled on, cut and similarly defaced, but what the heck is it for?
Backpackers know the lash tabs or "pig snout" is there to extend the pack's carrying capability by allowing you to strap another smaller pack, bedroll, etc. on as needed.
I wish I had known that as a kid so I could have strapped my dirty gym clothes on there instead of carrying them around in a plastic bag all day...