Question

How does the HCl in the stomach help in digestion?

Answer 1

This is a matter to pH in an environment causing a reaction to proceed at its optimum efficiency.

Explanation:

In the stomach we find enzymes such as pepsin whose job it is to break down the proteins we eat into amino acids. Like so many reactions, the rate at which pepsin can do its job is highly dependent on the pH of the environment of the stomach. It has maximum efficiency at a pH of about 2. The HCl in the stomach is needed to maintain this low pH value.
In a neutral environment, pepsin would be largely denatured (its structure altered) and it would be ineffective.