Because for fried chicken breasts to be tasty, the outer layer of crust must be crispy to eat, and if you only put flour in it, it won't come out crispy enough, and the outer layer of the chicken breasts will soften when it cools down, making it even less tasty. If you use starch, mix the flour and starch 1:1, the outer skin of the fried chicken breast will be crispy, and it will taste crispy on the outside and tender on the inside, with a very good texture.