Ok I thought of a better explanation lol

Does a backpack still work to carry things from A to B if it's not worn on your back? Is the perception of how you use it changeable? Can you technically use it by holding the top strap and still serve it's function? If you cut the straps off and carry hug style can you change the direction of use and still have it serve a function?

Now for storage purposes, if you want to access something, can you put it the opposite way and still see all the pockets? Can you access the stuff if you aren't facing it head on? Or is that perspective required in order for it to serve the use?

So we made up the way to describe it on the part of using it that has to be from a specific direction in order to keep things simple. There's no point in saying the back pocket, when in order to use it in that manner it absolutely has to be from the perspective where that is indeed the furthest back pocket.