Why does a chemical reaction occur turning 2 substances that are harmful into a necessary one for life, salt?

Let's assume you are talking about sodium hydroxide (NaOH) and hydrochloric acid (HCl). The pH of NOH is very high, and toxic because human body pHs are balanced at slightly above 7, which is the middle range of the pH scale. HCL has a very low pH. But when combined, these compounds neutralize eachother, and produce H2O and NaCl, water and salt. A balance is formed between the strong Acid and strong Base to create 2 neutral compounds.