The reason you would weave like this is if you sat down at a loom that was threaded to an overshot draft and there are no treadling directions anywhere. You don't have to cut if off the loom, you weave by reading the threading.
Take for instance this first photo. I have followed the 'blocks' from the edge and this next blocks I have done is showing me that there were seven threads, but one thread shares with each block, so I will only threw six picks on treadle three, (and of course I have used tabby). Always one less pattern pick than warp threads.
The next block is on treadle four and I will throw only three picks because there are four threads here.