Traditionally signs were non-illuminated. Mostly because signs have been around longer than electricity. With the onset of electricity marketers saw the opportunity to get more out of their signs but allowing them to illuminate, thus being visible for longer times as they could be seen at night.
With the arrival of neon, and the growth of Las Vegas, NEON signs became extremely popular because it was relatively easy to shape, is available in many colors, was relatively efficient. Today neon is not used as much as it used to be because while it is relatively efficient, t is less efficient than modern LED lights. Also neon, being a gas and powered by high voltage current, has a tenancy to ignite.
Today most signs are illuminated using one or other variant of energy efficient LED technology.