Drop sets how vertical the head is held when shooting. It is the distance between the top of the shoulder to just below the cheekbone. The larger the drop, the more upright the head. There are certain shooting disciplines that benefit from a head up position. Recoiless guns are not significantly impacted by large or small drop, but recoiling guns most certainly are.
Shooting arms invoke curious emotions. They are extensions of pointing and imply lethality. Using them requires balanced form to offset their weight. To send the projectile on it's way, one must exercise control over subtle body movements and pull the trigger with a smooth graceful micro-movement and then do this other unique motion, or really rather non-motion, called follow through. Follow through means keeping the crosshairs on the target during recoil.
Follow through is much easier done with recoiling arms, especially spring guns, if the stock has minimal drop so the recoil comes straight into the shoulder. Stocks with significant drop force the muzzle to rise during recoil, as the recoil is operating through an angle. This is an issue because a spring gun begins recoiling (and the muzzle rising) before the pellet leaves the barrel. Staying on target during recoil means when the pellet finally leaves the barrel, it's still pointing where you want it to go.
Additionally the action should be glass bedded to the stock to create a microscopically perfect fit. Slow motion photography reveals the action moves around in the wood before the pellet comes out, so having the steel and wood move as a unit makes follow through easier.
Correct top of comb to eye relationship and minimizing drop as much as possible is key to ease of accuracy with heavy recoiling spring guns. With minimal drop there is less muzzle rise on recoil because recoil comes straight back. The ability to stay on target during recoil is much more difficult when the recoil is operating on the big angle of a stock with a large drop. With my minimally dropped stock designs one can routinely watch the pellet fly to the target through the scope during recoil. You just can't get that experience with a severely dropped stock that allows the recoil to operate through a big angle and the resulting muzzle rise during recoil.
How the body drops into the positions that allow success determines how the stock is shaped to fit. We want a preponderance of the cheek in contact with wood so that there is a muscle memory that knows that contact spot every time. At that best point of contact, below the upper cheek bone but with most of the fleshy part of the cheek in contact with the wood, we should be looking down the sighting system. If it is iron sights, the top of the comb will need to be lower than the top of the forearm so that the cheek fits the wood correctly while looking down the sights. The head needs to move so low to get down to look through the sights that some drop at the butt will raise the head up higher, but at the cost of more difficult follow through because of muzzle rise during recoil.
Most of us choose to use scopes on our spring guns. Stocks to fit these guns will have the top of the comb at the top of the forearm so that when the eye is looking down the optic there is correct cheek placement on the stock. Additionally, there should be a slight slant in the cheekpiece so that it falls away from the cheek during recoil to avoid having the shot pushed off.