Thing about leather, it's made of crossweave strands. Push a round awl through it, it leaves a large hole, easy to sew, but the leather seals the hole up, simply because you aren't making a hole per se, you are just stretching the fibers out of the way leaving a hole. Is part of the reason you wet the leather when sewing, helps push the leather to seal itself. Hence the use of the lock stitch, leave the knot in the middle of the hole, the leather goes back to it's original state and locks the linked loops together inside.