Feed is the majority of a chicken’s diet, the staple. When you think of scratch, think yum… think treat… think “oh get in my belly.” It’s generally made up of cracked corn and other types of grains.
Scratch is tasty, but not really healthy (potato chips, anyone?), so it should be treated as a one-off, not a staple. And although scratch should be considered a treat, it totally encourages chickens to scratch through from grains (because it’s really that good), naturally turning litter and helping to maintain super clean conditions.
And who doesn’t want that?