You can't fix what you can't see. Monitoring tells you what's happening.
Metrics: Numbers over time. Request rate, error rate, latency percentiles (, , ).
Logs: Event records. Debug information. Structured logs (JSON) are searchable.
Traces: Request flow across services. Which service is slow?
In interviews, mention monitoring. "I'd add CloudWatch metrics on API latency. Alert if exceeds ms." This shows operational awareness.