Breaker clip in electrical panel

This clip was on the breaker for the water heater. I do not understand the purpose. Insight?

Google breaker lockout or lockoff device.

Was it labeled for water heater?

Yes, it was labeled water heater

Appliances Rated Over 300 Volt-Amperes or 1/8 Horsepower. For permanently connected appliances rated over 300 volt-amperes or 1/8 hp, the branch-circuit switch or circuit breaker shall be permitted to serve as the disconnecting means where the switch or circuit breaker is within sight from the appliance or is capable of being locked in the open position.