OK, I’ve been waiting for someone else to ask the obvious “dumb” question, but since none of you are volunteering, here goes: if it’s an AC signal, why does the directionality matter?
The idea being, since its AC its alternating, back and forth, forth and back, what's the diff? That about it? In a nutshell?
Okay so well first off in a house wired 120v the panel has 240v coming in with the breakers on the left connected to one 120v leg, the breakers on the right connected to the other 120v leg, and the 240v breakers taking up two spaces because they connect to both legs.
Got it? Okay. So even taking the back and forth point of view they are not all the same. One swings one way. One swings the other. (As Seinfeld might add, "Not that there's anything wrong with that.")
But what about 240v? What about the (admittedly rare) systems like mine where 240v steps down to 120v? There is no "leg" with a transformer.
Well the answer is even then the power coming in one wire is positive, the other ground or negative. In any case the AC coming into all components, the first thing it does is go through a transformer (to get whatever voltage the component uses) then to some diode rectifiers and then to some power supply caps. Because all components, even though plugged into AC, they all run on DC. But you knew that, right? ;)
So there's your flow.