HCl help in digestion of protein and help the immune system. HCl gives specific medium or certain pH level due to which certain enzymes activate. Enzymes such as pepsin or protease which breakdown the protein into its simpler form (amino acid). If HCl is not present in the stomach then the digestion of protein does not take place and it remain as it .
HCl also kill the microbes which is present or come during eating . due to its high concentration the microorganism is kill and in this way it helps to the immune system . If HCl is not present maybe we fall sick everyday.