LED lamps use DC power. The LED lamps that you can buy that screw into a standard Edison socket contain electronics to convert the AC line voltage into DC. This is inefficient -- generates heat and wastes power (though still not nearly as much as a traditional incandescent bulb).

By using power over Ethernet for their lighting, the datacenter can use cheaper, cooler, more efficient bulbs, and save a lot of money on the wiring too.

I can definitely foresee a future where new construction includes wiring for both line voltage AC and also low voltage DC. It could eliminate all the bulky transformers scattered around a typical house and save energy and money.