Tell us about networking monitoring!

Hello, I've been in the industry for 15+ years, but mostly small shops. Currently using Veeam One, and I'm about to set up PRTG software.

Anyhow, for whatever reason, we never talked much about network monitoring where I've worked. For those of you with more experience in this area, I would like to pick your brain!

You don't have to answer all three questions, just pick whichever questions for which you feel like you have a valuable answer.

1. What is one curious or interesting thing you have learned about network monitoring?
2. What is one basic and essential thing every tech should know about network monitoring?
3. What is one more sophisticated / advanced piece of wisdom you can share about network monitoring, that most techs often don't know about.

Cheers!

Josh

Please note I have a copy of this post also at reddit.com/r/sysadmin, and I made copy here based on another contributor's recommendation.