AC Versus DC Power
A century ago, most electrical devices, such as a fan or appliance, required 120V to operate. Today, 110V devices are in use, but technology has advanced to the point where electronics are prevalent and only need a low amount of DC voltage to operate; examples of this are mobile devices and even flat-screen displays.
. . . the bottom line is transformers are everywhere and there aren’t enough AC outlets to plug all of them in.