Stomach acid is actually hydrochloric acid. Stomach acid is very strong. It can even erode metal if you give it time. Stomach acid breaks down different aspects of food, especially proteins, The body needs broken down particles from food so that it could use it. But, the stomach is not the only thing that breaks down food - the small intestines does too, with bile!
So stomach acid makes food particles smaller so that the body can process it further.