Tesla's genius is that he figured out 120-degree three-phase power could be wired in a "delta" configuration without anything blowing up. This only requires 3 wires, as opposed to the 4 required by three-phase "wye" or or two-phase.

Large industrial loads (like substations) don't particularly need neutral for anything, so delta is used for economy of wire.

When you see a fourth or seventh wire on transmission lines, it is a lightning attractor to keep lightning off the conductors.

Delta power is great for high-power distribution; it is knocked down to either three-phase "wye" or split-phase near the customer. Industry has also been known to use 480 delta, though 480 wye/277 per leg is more versatile.

One advantage of 480 delta is that it can be supplied as an isolated system - sometimes important, say if it is going to be rectified to DC.

Transmission lines don't serve a diversity of loads; they serve substations. They don't serve any single-phase loads at all.

Tesla's genius is that he figured out 120-degree three-phase power could be wired in a "delta" configuration without anything blowing up. This only requires 3 wires, as opposed to the 4 required by three-phase "wye" or or two-phase.

Large industrial loads (like substations) don't particularly need neutral for anything, so delta is used for economy of wire.

When you see a fourth or seventh wire on transmission lines, it is a lightning attractor to keep lightning off the conductors.

Delta power is great for high-power distribution; it is knocked down to either three-phase "wye" or split-phase near the customer. Industry has also been known to use 480 delta, though 480 wye/277 per leg is more versatile.

One advantage of 480 delta is that it can be supplied as an isolated system - sometimes important, say if it is going to be rectified to DC.