To study interstellar gases, pulsars and quasars in astronomy and primarily communications and tv in everyday use.
Astronomical objects emit many types of waves on the EM spectrum, not just visible light. Because these waves are so long, they're not as susceptible to atmospheric influences such as clouds and rain. This gives radio astronomy an advantage over optical astronomy since the radio telescopes can be based on Earth and can work in day and night.
Radio Astronomy is useful to probe into parts of universe that has little visible light such as determining the gases in interstellar medium. It was also responsible for the discovery of pulsars, which are spinning neutron stars, and quasars or galaxies with supermassive black holes at their center.