Extraterrestrials might use radio waves instead of light waves, for several reasons. First, vast clouds of dust and gas in space absorb light waves, blocking our view of much of our galaxy. Also, powerful beams of light must be very narrow, like a laser. They must be aimed precisely at the viewer. But nonvisible waves, like radio waves, can easily pass through space dust.
Radio waves can also be very wide.
One radio beam can cover a lot of sky. What's more, visible light waves are only a small fraction of the entire spectrum of electromagnetic radiation. Our eyes can only see this narrow band of light. Using the proper instruments, however, we can detect signals in other parts of the spectrum, including radio waves, x-rays, gamma rays, ultraviolet and infrared light. Astronomers use radio telescopes to observe radio waves coming from stars, nebulae, and other galactic phenomenon.