Interconnects, some have directional indicators, why?

Arrows on all other kinds of cables -- XLR, speaker, power, and data -- are utterly meaningless from an electrical standpoint, since audio signals (and wall voltage) are alternating current and interact with conductors the same way in either direction.

I'm only sure it is useless for XLR cables, since you can plug them only in one direction.

Kalali, you can do that, but good electrons, that you paid for, might leave the cable, being replaced by just plain cheap electrons.

Current in the wire is a flow of electric charge. Wire creates magnetic field but transports charges and not the energy. Energy is transferred thru magnetic field from the source to the load.
The load has some voltage drop in it, hence electric field. Together with magnetic field, this brings the energy in. Same way, the source generates voltage and, together with magnetic field, this brings the energy out.
Direction of electromagnetic energy flow is determined by the Poynting vector, E × H, and depends on both voltage polarity (through E) and current direction (through H).
As I understand it, even in DC circuit energy flows from source to load thru the space in direction of Poynting Vector.

Jim, electromagnetic field would be contained in conduit or even braid.  Very often explanation of Poynting Field brings example of coaxial cable, showing that whole energy flows thru dielectric between the wire and the shield.  As for Boulder 3060 amp - I don't know.  Perhaps somebody else?  Al?