It's so that you can drive more monitors than a single GPU supports. For example, my T430 with discrete graphics could drive 4 monitors simultaneously while the

It's so that you can drive more monitors than a single GPU supports. For example, my T430 with discrete graphics could drive 4 monitors simultaneously while the iGPU model was limited to 2 (3 unofficially). It became so annoying at one point that I just replaced the motherboard with the iGPU version. Apparently now nvidia optimus support on Linux is improving somewhat, but having had to deal with this and other issues for years, I'm probably never getting an nvidia GPU again unless I absolutely have to.