I guess the most elementary way to describe the principle is that the board typically is very inhomogeneous vertically, because each player starts on his own side of it, while horizontally it is more or less the same anywhere: some stronger pieces behind a wall of Pawns. So it matters much more at which rank a piece stands, and whether it can easily move there, than that is matters which file you stand, so that you can be happy where you already are.
I guess the most elementary way to describe the principle is that the board typically is very inhomogeneous vertically, because each player starts on his own side of it, while horizontally it is more or less the same anywhere: some stronger pieces behind a wall of Pawns. So it matters much more at which rank a piece stands, and whether it can easily move there, than that is matters which file you stand, so that you can be happy where you already are.