@ColonelCrisp
Why? It's better than trying to stay incredibly quiet while you sneak up on someone. Your body occludes sound, which is what happens when you get that feeling of "there's someone nearby/someone behind me" and there is. You're not psychic, but tiny ambient sounds you were hearing clearly from a distance are now being blotted out from a specific direction. Which, as anyone knows, often makes you check out that 'feeling'.
A loud shout just as you come up on someone will panic them in most cases, causing them to scramble, or not know which direction to look in. Or both. This is why oriental martial arts and samurai-training often included the 'kiai', a specific martial shout meant to intimidate, but also confuse and disrupt the opponent. It works even better during a surprise attack affording that you aren't dealing with someone wildly more experienced than you are and that you're doing it from close enough up. Too far away and you might give them time to react, which fucks you.