The most useful feature of the gauge: you can see whether the carb is using the off-idle or power circuit. Generally, a carburetor shifts to the power circuit at low vacuum (more accelerator) and/or high rpm. You can keep it in the off-idle circuit by keeping the vacuum above 10" or so. The exact switch over point depends on the carb design.
I am actually planning to install a vacuum gauge in my dad's car; there is one lying around in my house. I want to see how it works for other people, to collect more data