What is echolocation used for in whales and dolphins?

Communicate and to navigate.

Explanation:

Echolocation. Toothed whales (including dolphins) have developed a remarkable sensory ability used for locating food and for navigation underwater called echolocation. Toothed whales produce a variety of sounds by moving air between air-spaces or sinuses in the head.

Echolocation allows dolphins to "see" by interpreting the echoes of sound waves that bounce off of objects near them in the water. To echolocate objects nearby, dolphins produce high-frequency clicks.