REI Sleeping Bag Slip-Not Straps exist for one single reason: To hold your sleeping bag down on your sleeping pad. This is critical. If you don't strap the bag down to the pad, every time you turn over you will roll off the pad. This means, you'll be sleeping on solid ground. And even the cushiest sleeping bag won't protect your aching bones.
The stretchable straps have smooth plastic hooks on both ends that won't tear or abraid your bag or pad. Hook one end through the loop on your bag, lift the pad and throw the other hook to the other side of the pad and hook it on the other side of the bag. Repeat this with the second strap on the other set of loops on your sleeping bag and you're set. There! No more sleep rolling.