Our stomachs secrete hydrochloric acid. what function does this hydrochloric acid serve

to digest food and to kill microbes

Food that we eat is mostly not sterile and contain some microbes. A strong acid can destroy the microbes and prevent infection in the gastrointestinal system. Acid also can help destroying food, make it much smaller. A Smaller piece of food is easier to digest since the area that able to do chemical reaction will be bigger.