This knot traps the two lengths together. If you just use a reef knot, a) you'll have more slack, because there's no pulley mechanism to take advantage of to ti

This knot traps the two lengths together. If you just use a reef knot, a) you'll have more slack, because there's no pulley mechanism to take advantage of to tighten the string, and b) it's easy to accidentally hang the picture by only one of the lines, so the slack has twice the effect, and the picture hangs lower than it otherwise would.