As the other commenter said, it would probably be for post-workout. Chocolate milk has fats, proteins, sugar, and a variety of micro nutrients. As far as I’m aware sugar is actually pretty important in liquid recovery drinks because it encourages the cells to absorb the sugar which allows other things to also join the fray. Fats and proteins also have numerous positive affects on muscle gain which I think is broadly known to some degree. IIRC this is the reason Gatorade and like drinks have the amount of sugar they do (plus palatability).

This is not a statement of fact and should be verified by primary sources*