The new GP chemical pumps that are rated for higher pressures have a built in shut off valve. I can only figure the reason is so the diaphragm doesn't get damaged under those high pressures. I can't see any other reason for it really. A hole can develop in the diaphragm so it seems logical to me that shutting it off while working with higher pressures would make the chem pump last longer w/o problems. There is no way you can starve the chem pump. It works totally off the pressure of the high pressure pump.
I replaced almost all of my chemical feed system last year including changing to the new style GP pump. Mine is the lower pressure model that I think is rated for 1500 psi because it has no shutoff valve. Again, I still think that's stretching the truth. I haven't had any problems yet caused by the chem pump itself, but I don't do a lot of tile so I'm typically way under the max pressure limit and my machine tops out at around 1,000 psi anyway. They do make a higher pressure model that's being put on machines more geared for higher pressures that comes with a shut off valve already installed. Personally I think it's a good idea if they would do the same for the 1,500 psi model as well.
Here is a .pdf of the newer General pulse pump. It is supposed to be rated for 1500 psi w/o the shutoff valve engaged and up to 3,000 psi with it engaged. Personally I think it should be engaged if doing anything requiring higher pressures. If I run into trouble with my new pump I may put something similar on mine.
http://generalpump.cazarin.com/wp-content/uploads/2017/10/100152-PulsePump.pdf
https://www.generalpump.com/product/100152/