A farmer who sells eggs has to use feed for his chickens. If he let them "just" eat grass/seeds/bugs they probably would not be daily layers and he wouldn't have enough eggs to sell. Chickens have to be very well fed to lay every day.
It makes a huge difference in the nutritional quality of the eggs to allow the hens to graze in addition to their regular feed. The yolks will be more orange and the eggs will taste better and have a more optimal nutrition profile.
Don't be afraid to eat animals that eat grains IF the animal was meant to eat that stuff! I don't like eating grain fed beef but grain fed chickens is ok with me because as far as I know, chickens eat a more diverse diet than cows and haven't adapted to a grass only diet like cows have (ruminants).
It sounds like you have a great farmer to work with. You are lucky to have that resource. He is helping you learn more about food and farming and that is a great thing!