because the screen of your laptop is directly connected to the IGP of your CPU (the HD intel), not the same on desktop pc's when the dedicated GPU have their own slot to plug the screen (VGA,DVI,HDMI....).
a dedicated GPU work on laptops like a co-GPU to transfer heavy task from the Intel IGP, that's why you can't disable the intel HD from the bios... eh it's hard to explain with my english.
but you can set the default from the Nvidia Control panel:
3D Settings->Manage 3D Settings
Tab "Global settings"
Preferred Graphics Processor
Select : High performance NVIDIA processor