This is a great exercise for shoulder width. With this variation, every rep begins from a dead start. This forces the muscles to go from zero contraction to full contraction instantly. It also eliminates cheating. Just briefly rest the dumbbells on the bench before starting each rep.
Unless you have ridiculously short arms, you'll need a booster seat for this one. A few plates will do the trick.
As with all lateral raise variations, don't focus on lifting the dumbbell up, but rather "pushing it away" from you.