Every other vacuum-related part in this category — the hose, the regulator, the belt — does something to the vacuum itself. The Vacuum Gauge doesn't; it simply shows you, in real time, what the vacuum pressure actually is at any given moment, which is what turns troubleshooting from guesswork into something you can actually diagnose. Without a working gauge, you're relying entirely on milking behavior (cows kicking, slow milk-out, inconsistent timing) to notice a vacuum problem — with one, you can catch a gradual drift before it gets that far.
Checking the gauge regularly as part of routine maintenance, and comparing it against your machine's known correct operating pressure, is one of the simplest habits that catches problems early — a belt starting to slip, a regulator drifting out of calibration, or a developing leak will usually show up as a gauge reading that's off from normal well before milking performance visibly suffers.