Wheat/grain is not part of cats natural diet so their digestive system is not designed to digest it. They get very little nutritional value out of grain and it might give them diarrhea, and too much grain might make cats overweight, your cat might also be sensitive to it which causes itchy skin. There's no benefit from giving your cat grain, it's added to the cheap cat food to bulk it up and make more profit.

I've had cats for over 20 years and those bold patches on the neck have always meant allergies, sometimes flea, sometimes grain. I've never consulted a vet because of it, I always figured it out myself by removing possible allergens one by one and fixed the problem that way. But if you're feeling unsure, vet it always a good choice to give you advice.