How can you explain the division of the stomach in birds into two sections, glandular and muscular?

In the glandular part of the stomach, food is digested chemically, that is, with the help of the glands that are present there. In the muscle, the food is crushed. With the help of muscles, food is set in motion and, thanks to friction against solid particles that the bird swallows (pebbles, shell fragments, etc.), it is brought into a crushed state. Since birds do not have teeth with which you can grind food, and with the help of a beak it is impossible to grind it to the desired state, the presence of these sections in the stomach is extremely necessary for birds to digest food.