Vacuum gauge

Forgive my ignorance, I am going to put a tach and oil pressure gauge in a pod on the drivers front window post. I was thinking of adding the vacuum gauge to the pod.

My question, I have a vacuum gauge that I can hook up for testing, but why would I need to monitor the engine's vacuum while driving?

I figure that if I need one I would add it to the two I plan on adding

Thanks in advance for your feedback