What is a circuit breaker actually doing for a Node service, and when would you bother adding one?

So a circuit breaker basically wraps your calls to a dependency and keeps track of failures. In the closed state, calls just pass through normally. When failures cross some threshold, the breaker opens, and after that calls fail immediately without even touching the dependency. That gives the struggling service room to recover, and your users don't sit through the full timeout wait. Then after a cooldown it goes half-open and lets one trial request through. If that succeeds the breaker closes, if it fails it opens again. You'd add one in front of any remote dependency that can fail or hang, so payment providers, third-party APIs, internal services, all of those qualify. Most teams just reach for a library like opossum rather than hand-rolling the state machine. And combined with retries and timeouts, it stops your service from repeatedly hammering something that's already down.