A boost gauge measures the pressure being produced by a turbocharger in an engine’s intake system. It tells you how much “boost” — compressed air above normal atmospheric pressure — is being forced into the engine.
A boost gauge helps drivers:
Monitor turbo performance 
Detect boost leaks 
Spot overboost problems 
Tune performance safely 
Know when the turbo is working efficiently 
On modified/performance cars
In tuned turbo cars, a boost gauge is especially important because:
Too little boost can mean leaks or turbo issues 
Too much boost can damage the engine 
Stable boost helps verify tuning and wastegate operation