Juniper supports a large variety of hardware platforms, each enriched with the functionality of JUNOS, enabling customers to operate various applications: from single-domain enterprise networking through to large-scale, carrier-grade, service provider multi-tenanted networking. Regardless of where you are on that scale, once you've used the rich JUNOS functionality to solve a particular network problem, nothing stays constant. Unsolicited change is inevitable, whether it be caused by growth in network traffic and the need to upgrade, or outside intervention in network links causing logical topology changes.
An invaluable aid to any network engineer's toolkit is a rich capability when it comes to monitoring variables and metrics associated with the JUNOS device. Is the device operating in the designed tolerances? Are there any outstanding cautions or warnings...