You learn non-verbal coms in the military is pretty standard. Sometimes you cannot talk
1) for tactical reasons - you don't want to be heard (eg if the people inside the house could hear your, they'd shoot you all through the wall)
2) for visual reasons - much work is done under the cover of dark
3) for logisical reasons - you cannot hear because it's too loud and/or the person is behind you (say, in preps to parachute from the airplane, you tap the guy ahead of you to tell him your ready)
4) for psychological reasons - easier to go in the door or jump from a plane when you know your buddies are immediately behind you
On the stack they also have their feet touching and/or legs intertwined to reduce space and for body contact so you can keep your eyes up, not looking for your buddy.