The double use of the post-pump shutoff is that when you do unplug the pump for maintenance, you can shut off the vale and stop the backflow, then just let the tank drain down through the overflow.
However if your tank is set up like this you need to do a power outage test, unplug the pump and let it drain to make sure you don't flood the sump. Sorry for the hijack, just my 2 cents
My tanks: 120 Reef w/L2 Algae Scrubber, 60 Reef Pond w/floating Algae Scrubber, 40 Breeder Reef w/L2 UAS Tester Algae Scrubber
I maintain: 144 SW Reef w/L2 Algae Scrubber | 200 SW Reef w/L4 Algae Scrubber
Special knowledge: Algae Scrubbers, Acrylic Fabrication