When a PoE cable is used to transmit power, it uses two wires for DC+ and two wires for DC-. In contrast, a simple light bulb powered by a battery circuit uses just one DC+ wire and one DC- wire. Why two wires for each polarity in PoE? Is it because the gauge of the wires are very thin and may not reliably transfer power over larger distances?
This is because the wires are used for power AND for data at the same time. The power is carried common mode in each pair, one pair for the supply and one for the return, while the data is carried differentially. There is also the added benefit of doubling the current carrying capacity and halving the resistive loss. Using a relatively high voltage also helps send the power a long distance.