To state that wire has "directionality" is to toss aside the laws of quantum physics. First. the conductivity is proportional to current density and electric field, irrespective of the physical dimensions of the conductor. Second, the net drift velocity of the electrons is zero. If one direction in the material favored the other, there would be a net change in velocity in that direction and every wire would have an electric charge.
Energy transfer through the wire is by electric and magnetic fields, which move electrons to higher velocity or energy states. Electrons do not move from one end to the other, as water through a pipe, instead electrons are bound in a Fermi surface, which represents the energy state and momentum of the electron. When an electric field is applied, it "shifts" the Fermi surface away from the field and displaces electrons only on the surface of the field, accelerates those electrons on the displaced side to a higher velocity state and scatters electrons on the other side. Photons, lattice vibrations and impurities slow down the scattered electrons and cause resistance. This creates the space charge that propagates the applied electric field along the wire, and transfers electrical energy.
If you were to look at one point of the wire in an AC field, the Fermi surface distorts back and forth along that point, and not electrons flowing by like water. Again, the Fermi surface is not an actual surface but a representation of electron momentum. To say a wire has direction, you have to accept that a) the Fermi surface is not equally displaced by the same magnitude E field from both directions, and b) the scattered electrons on the other side see less (or more) lattice vibration and impurities for some reason.
So, why the arrows on cables? Mine have them because MIT placed network boxes on the wire and the direction of the signal is impacted by the passive devices of those networks -- which is audible if installed backwards. But putting arrows on plain wire is just decoration.
