External links provide integration between Operations for Applications and external systems. If you use logging systems, such as ELK or Splunk, you can easily construct a meaningful URL to navigate from Operations for Applications to a log entry.
Suppose while analyzing metrics data, you find an anomaly, such as an unexpected drop in transaction rate. You want to navigate to logs to look for entries that could explain why the transaction rate drop occurred. External links allow you to click through from an Operations for Applications series directly to a related entry in your logging system.
External links are general purpose: you can link through to any type of system, not just logs.