On the other hand when you have a reasonably complex deployment it's easy to get swamped with dashboards showing CPU, Memory, I/O, application-metrics, signups, active users/sessions, etc.

Instead it's nice to think about how you can express the state of a complete system as a single number. It might be you divide active user sessions by database-connections, and then scale by memory capacity.

But as a single digit you can then get used to normal ranges, and have it always visible somewhere obvious. A single number won't show details, but when it changes you can go look at the specific metrics. It's a cute shorthand, and it can work well as a basic "are we normal" check.