Coaxial wiring just means that instead of two copper wires running side by side, like a shotgun barrel, one cable goes down the center of an insulated tube, and that tube is surrounded by a mesh of wire, which is then intern insulted. It’s stronger and sleeker than the side by side thing. Sold by the foot, get more coaxial wire for various lighting projects.
Like, say you are trying to wire a Schmidt Fender Mount Taillight on a Rivendell and you suddenly realize: "holy moly! this bike is loooooong!" So you need to buy more wire to splice in to make the connection to the headlight. While you're at it you might as well get a pair of male and female Schmidt coaxial plugs to make the connection sleek as possible and give you a nice break away option if you ever decide to remove those theoretical fenders.