having x-monitor installed. what information (useful) is gained by the boost guage?

 

The higher the boost, the harder the engine is working, I.E. more fuel, higher EGT's.
Driving with lower boost numbers will get you better fuel mileage.

 

The boost gauge will also be usefully to detect a leak in the intercooler and associated pluming.
You know what is normal. If it ever falls below that, you have a problem.
Also, if you plan to bomb the truck, it would be nice to know how much boost you are running. (too high will blow a head gasket)