Observations at radio wavelengths are highly informative because they allow us to see things that are not detectable in the optical. Radio waves pass easily through gas and dust clouds, so stars or galaxies that would be invisible to an optical telescope can be seen at radio wavelengths.
Certain regions of the Universe contain molecules that absorb or emit only at radio wavelengths. This is the case with neutral hydrogen, which emits at a wavelength of 21 cm. By observing this emission line, velocity and temperature maps can be made of these regions.
Example of how observations at radio wavelengths and in the visible complement each other: (a) the Galaxy M87 with its jet of subatomic particles seen in radio waves by the VLA, and, (b) the same jet seen in the visible by the HST. The radio image reveals the presence of a giant structure, undetectable in the visible, whose emissions in radio and x-rays are produced by jets of particles emanating from the Galaxy. Credit: NRAO/NSF/STScI.
In addition, some objects emit strongly in the radio domain but weakly in the visible. This is the case of pulsars (neutron stars spinning at fantastic speeds), of quasars (young galaxies that are so distant that they appear as a single star, but which are among the most energetic objects in the Universe), and of radio galaxies. All these objects can only really be detected in the radio domain.
For all these reasons, radioastronomy is an essential complement to optical astronomy in furthering our understanding of the Universe.