GE D11, LP Inlet Pressure Control SP?

So, I operate a 2x1 plant, self learned, migrated from coal. We have two GE7FA.05 CT's on NE HRSG's, to a GE D11 steamer. The HRSG's are three drum, LP, IP (with reheat), & HP. GE Mark6, and ovation for the BOP. The D11 has a low pressure steam inlet / reg valve that receives additional low pressure steam from the back end of the HRSG's. The SP on the Mark6 HMI is adjustable from 42~72 psi. The pressure reference is taken from the HRSG, or upstream side of the valve. The SP can be changed at any load, willy-nilly. (The LP steam is not used for any other process). We have no reference in the manuals that speak to its function, why it's there, when to use it, adjust it, but only that it exists where it is. Nothing more. Changing the SP does nothing for load. I can only guess what it's for but I don't want to bias any response. I've been asking the question for three years with no response. (Yes, I've asked ALL of THEM).

Even if someone has some "play time" experience they would like to share, I'm all ears!