It's not only for trapping. Say you insert a double edged blade into bad guys body then try to lever it, either to move him or to make a really nasty wound. Since both sides are cutting it will kinda spin in place. Also the blade can get embedded in bone if you try to lever in certain places. With the dull cutout flesh, muscle, tissue bunches up there providing a fulcrum with which to lever and/or snap the other end out. You can also place the dull portion against bones (ribs, vertebrae etc.) too for the same type of levering action and the blade won't bite and get stuck in the bone.