I am not going to try and conduct a physics class here but will just try and briefly describe what "recoil" is all about. We all know that actions produce equal and opposite reactions. But with firearms, it is not just a rearward motion. there is also an angular motion induced due to the buttstock being lower than the barrel - So the interface of a shooters shoulder and that stock becomes a pivot point and will create this angular motion out of or in addition to the rearward motion created by firing the gun. If everything lines up straight with the firing point, very little angular motion will be induced and the recoil will be rearward into the shoulder. This is why many of the modern rifles feature straight stock designs (think AR). The Ljutic Space Gun was an in-line target shogun and a couple of his other designs were close.
So the focus (on controlling the angular motion) becomes the interface between your shoulder and that buttstock (and your head / face) Sometimes, the toe of the recoil pad will hit your shoulder chest area first and will really assist the angular motion. This is why folks will talk about the need to change the "pitch". Sometimes there is a need to add an adjustable buttplate so "toe out" can be provided. This is all about getting that buttstock to fit into your shoulder pocket evenly. Those concave recoil pads are not for every gun or every shooter.
As stated by Ram Rod, a shooters stance or pose and the way a shooter mounts the gun, neck length, facial configuration can all contribute to increased angular motion and why comb/stock offset can mitigate at least some of this.
Keeping a solid cheek weld is not possible if the gun is rotating into the shooters face - subconsciously a shooter is going to try and get his face out of the way and then a bad situation is going to get worse as the gun now has a running start. This is why people install cheek pads and soft combs. They were unable to adjust their stock for a better fit and so they just put a buffer between the comb and their face.
I am including a pic of one of my guns to illustrate toe out and offset - PS -
this is a rocker pad