BLB, are you referring to the Mike Epstein clips? Softball
If no, then I am not aware of the deltoid drill. If yes, I have a theory, but I'm not sure if it's a good theory.
Epstein is still developing these hitters. The muscle memory from many years of a flawed swing with bat drag is difficult to overcome. By starting the hitters with the elbow already in the slot position, they are less likely to go into bat drag.
Starting with the bat on the deltoid takes the arms out of the equation. Then, the body mechanics can be the focus.
As MVLA said, many hitters have the habit of arm extension toward the catcher being the very first move their hands/arms/bat makes (or some other unwanted first movement). Resting the bat on the deltoid keeps the arms/hands/bat more connected to the movement/power being generated by the other parts of the body.