Typical scratch grains are mostly corn, with some other stuff mixed in. It's not vey high in protein, and generates a lot of calories that make the bird warmer. Fabulous for the winter weather.
Grandma had free range chickens, they got lots of protein from bugs, lizards and worms, even the occasional snake. The scratch was pretty much all there was in the way of prepared feed at that time. So with the free ranging, scraps and scratch, they had a pretty balanced diet.
These days we have all this commercial food, supposedly well balanced, and scratch works better as a treat.
I have some chickens that free range, some that don't. In 16 years I have only lost one to illness. Don't know if that means anything or not.
I free feed commercial lay pellets, and give about 15-20% scratch during the winter. During the summer, they get lay pellets free feed, and get to free range. No scratch, but lots of scraps from the garden.
There many opinions about this, just decide what works best for your chickens and go with it.