Howdy fel10,
Battery savings is the primary reason, and maybe temperature. Running the integrated graphics only, will run cooler than the dGPU. I am not sure about macOS, but under Windows (not BootCamp on Macs as the dGPU is the only available option) you might want to use the integrated video to take advantage of Intel QuickSync technology, but other than that, it is just for battery life 🙂 Good luck!
Rich S.