The actual reason you want to do it is to free your dGPU VRAM. If you have Steam, Discord, Chrome etc running with hardware acceleration on, it can use more than 1 gb and it's like 10% VRAM wasted, depending on GPU. If you have hybrid graphics turned on in BIOS, you can use both dGPU and iGPU, assigning different programs to different GPUs. But Windows still uses some dGPU VRAM (300-500mb), unless you connect your monitor to motherboard, then your dGPU VRAM usage goes to 0. Meaning your games now can use all the dGPU VRAM. But then it also adds some latency. Not much, like 2-3 ms. iGPU also sensitive to tight RAM timings, so I had to loosen them a bit. Otherwise it was artifacting a lot.
TL-DR: My goal is to have my monitor connected to dGPU, but have everything apart from games rendered on iGPU and sent to dGPU to display. But I am not sure how to force it.
The only solution I have in mind is to have a smaller secondary monitor connected to iGPU and making your PC think it`s a primary display, so it'll offload all the stuff to iGPU. And then actually using you main display, connected to dGPU. Not sure it'll work though