Why use a relay vs a circuit breaker or switch ?

By definition :

RELAY : An electromagnetic device for remote or automatic control that is actuated by variations in conditions of an electric circuit and wich, in turn, operates other devices (such as switches) in the same or a different circuit.

CIRCUIT BREAKER : A switch that automatically interrupts an electric circuit under an infrequent abnormal condition; e.g, a fault condition such as an overload or rupture of either high voltage or high current or both.

SWITCH : A device for making breaking or changing the connections in an electrical circuit. Usually mechanical and operated by hand.