Relays need to know the current magnitude – either for metering or protection. A current transformer (CT) fulfills this role, stepping down thousands of amps to (typically) 5A which is then fed to a relay.
Disconnect switches establish isolation points to enable equipment maintenance. They are installed on both sides of the circuit breaker, high-side of the transformer, and on the line exit. Switches cannot and should not be used to interrupt load current or fault current. A switch operation takes place after the circuit is de-energized.