The way it was explained to me during training is that if you can't do "pointless" things in a calm and controlled environment, how can you be expected to do it during war? As a soldier you are told to do things you don't understand all the time but they very well may be important despite your lack of understanding. Drill is the embodiment of that. It's "pointless" traditional activity but it represents the core of military philosophy. No matter what you're asked to do, you do it together and you do it well.

These guys are obviously above and beyond regular drill, but the principles for "why" are the same. A good soldier acts as part of the group and they do it without hesitation.