the back gauge tells you the pressure of the LPR - it helps when fine tuning the gun. Most people take the gauge off when not testing the LPR pressure but it is really just personal preference.

That'd be my baby.  As Lights Out mentioned, it's for fine tuning.  I also use the rear gauge to diagnose any problems I may encounter as well.  If my velocity starts spiking, or if I start to experience shootdown, I can see at a glance what the LPR is doing, as well as the HPR.  And it's a quick way to retune if I ever have to tear the LPR apart, rather than counting turns or guessing.  I can just look at the gauge and get the LPR spot on.