Gpu's can fail. There's countless posts on 'no video output', even with a brand new pc, and not necessarily because anything is broke, but sometimes just because of a pcie setting being wrong. But that means you can't then get into bios, to fix the issue. Having an igpu as a backup, seperate from the discrete gpu is nothing more than an insurance policy, it's there if you need it. Considering you'll probably own the pc for the next 5 years±, $40 worth of insurance is a no brainer. You'll spend 10x that amount just in Starbucks or McDonald's.
PC wisdom 101: it's better to have and not need, than to need and not have.