A heater keeps a tropical tank at a stable temperature; tropical fish need one, while coldwater fish such as goldfish and white cloud minnows do not.
Stability matters as much as the exact number — a good heater is thermostatically controlled and correctly sized for the tank volume. A separate thermometer lets you confirm it is actually working.
A stuck heater (either off or on) is a common cause of sudden fish stress, so it is worth checking during routine maintenance.