Why feed grain? Birds do love it. I hate calling it a "treat" though. I accept, however, that "supplement" is a lot more unwieldy to write than "treat."
But to the question: I feed grain partly because it develops the gizzard, which makes a stronger bird. Feeding chickens only mash or pellets is akin to humans consuming only a liquid diet. May not matter to most, but I raise game chickens, which are and should be more like wild birds.
I feed grain because I love blending the different types. For me, a pellet is used mostly as a handy protein boost and ensures covering amino acids like lysine and methionine.
I dislike cracked corn. Whole corn is a viable seed. It has life in it. Once cracked, that life dies, and the particles steadily oxidize and lose feed value. Some experts agree with this widespread, commonsense lay opinion among old-time chicken folk, but there's no reward for research into it. Industrial farms don't feed scratch.
Having said all that, I have been feeding some cracked corn because it's in a quality grain and pellet blend I use: Bluebonnet Conditioner. I like the whole popcorn in it best, but I have never seen cracked corn like this. It is small chips that are bright orange. It is like bits of the shell only. The typical raggedy cracked corn pieces and white dust aren't there. The feed is very clean, so maybe the large bits and dust are winnowed out and used for pellets.
I wish I could find a source of the smaller kernel Nutra Dense corn that doesn't involve exorbitant shipping. It's much higher nutrition, including more protein, lysine, and oils. Heirloom and many open-pollinated varieties of corn have higher feed value too. Enhanced nutrition is in great tension with yield, so most corn we can readily buy is a high-yielding carb source somewhat lacking in nutrition compared to corn bred for a broader range of nutrients.
Here's a company that sells good whole grains, including Nutra corn:
https://dmfnaturecenter.com/product-category/seeds-grain/grains/
Amazon offers some small bags from them of mostly Nutra corn. I think it is ostensibly a pigeon mix.