Use case?
Pretty much it's for whatever I want to do with it. I will admit that at one point my work Mac had three displays and I was not to be outdone by that at home. So, I stuffed three video cards into a PowerMac G4 Quicksilver and got six displays. That followed to a PowerMac G5 with two video cards and then to my Mac Pro with two video cards.
It stayed at six displays and two video cards until last year when I acquired additional 30" Cinema Displays and an additional video card became necessary to drive them. That made 8 displays. But it also bugged me that two ports were not being utilized on that third video card. So, I added two additional monitors.
This is a Early 2009 Mac Pro running Sonoma via OCLP. A 4,1 MP that I upgraded to a 5,1. Its got two flashed NVIDIA GT640 cards (which originally allowed me to run Mojave on it) and I added an NVIDIA NVS 510 last year. So, three video cards is how I accomplish this.
I was not aware there was some sort of limit somewhere.
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