The word prescription is not right. It's a recommendation. Most vets have this built into their business model and make some extra money off of it.
Often it's the right thing to do depending on your vet. Andy and Marco (non-diabetics) had frequent urinary tract infections and when we switched them to Hills SO the problems stopped. Andy went missing for 88 days and lost almost 1/2 his body weight so he was put on a kitten weight gain formula. There's no prescription needed, I can walk into any vet and buy it.
My problem is with big box stores that make medical claims they're not authorized to make. Weight loss is generally the biggest lie. Most (not all) cats who get overweight, not obese, are going to stay that way. They've been wild animals for a million years and now we keep them inside with nothing to hunt for.
Before there were holistic or grain free foods I had two brothers that ate Safeway Low Ash canned food, basically pig slop in a can. One lived to 17 and the other to 23. Sometimes it's more genetics and good luck than overpriced food.
There is no such thing as a stupid question here, we're all in the same leaky boat.