Want the short reason? Resting your barrel on an object, like cover or a support, makes your shots miss. Barrels have flex to them, and placing the rifle on the barricade moves the point of impact off. Good enough reason? Good. Want to know more? The video explains it well, albeit a little long winded.
Thankfully it’s a 100% avoidable problem. Simply use the handguard when resting on a barricade or other bit of cover.
Want to take it a step further and not only prevent the barricade from throwing off your shots, but use the barricade to be more accurate?
Barrel flex in action