Using the sand itself and putting it on the clubface will allow you to accomplish a number of things with regard your clubface control, swing path and tempo that will hopefully lead to better bunker play.
Scooping sand onto the clubface, the first thing it does is it helps you feel what an open clubface should feel like. It also allows you to feel what the proper rotation feels like taking the club back.
Finally, it helps with rhythm and tempo because if you go too fast or too slow, the sand will fall off the face, so really what you’re looking for in this drill is to toss that sand over your shoulder at the transition.