An indoor-only cat is very different from an outdoors cat or even an indoor/outdoor cat. Think of all the constraints and expectations we place on indoor cats:
1. we don't want their litter boxes to stink
2. we don't feed them varied meals -- same thing over and over again, and any nutritional deficiencies will eventually become apparent
3. we want their coat to be shiny and soft
4. we don't want them to get fat but we don't give them enough exercise
5. we want them to be evenly energetic -- not full and bloated during the day, running around crazy during the night
6. we don't want our cats to be sickly or have bad breath
7. we expect long lifespans
8. and so on!
You will also see kitten, adult and senior varieties of the premium brands. The kittens are still growing and the seniors have slowed down a step, so just small adjustments in their food supply to compensate.
If it matters, the Nutro brand would be my first choice of what to feed a cat unless it had special needs that were better met by a different food. It is a good "all-around" food that is premium quality but still readily available in any pet store.