Vacuum Gauge - Why?

Listers,
Have a set of additional gauges that came with my car.  One is a vacuum gauge.  
My non-knowing question is why would you need this?  I obviously understand why 
you need oil pressure, water temp., even the ammeter that is included in this 
additional gauge set.  However this vacuum gauge escapes me.

Thanks,
Michael Lott
(usually lurking and learning)
Compton, CA