H+ vs HCl in the gastric juice

Parietal cells in the stomach are known to expel $\ce{H+}$ ions into the gastric juice to keep the pH acidic. When $\ce{H+}$ $\ce{[H3O+]}$ can fully function as acid in the gastric juice, why do cells combine $\ce{Cl-}$ to form $\ce{HCl}$ in the stomach? 


