You can put it in to make the beef tender faster.

Beef has more connective tissue than pigs and mutton. The connective tissue is mainly composed of collagen protein, which makes the muscle tissue particularly tough. Therefore, when stewing beef, it takes a long time to heat it until it becomes tender. However, when the collagen protein of beef is heated in an acidic environment, it will accelerate its decomposition into soluble and soft gelatin. Therefore, when stewing beef, adding an appropriate amount of vinegar will not only speed up the cooking of the beef, but also have the effect of removing the mutton and improving the flavor.