What is the purpose of garnishing cocktails like the Manhattan and Rob Roy?

Garnishing cocktails such as the Manhattan and Rob Roy serves to enhance the flavor of the drinks. These garnishes, which often include twists of citrus peel, cherries, or other complementary ingredients, provide aromatic elements that elevate the overall tasting experience. When consumed, the aromas released by the garnish interact with the palate, contributing additional dimensions to the cocktail's flavor profile. For example, the bittersweet flavor of a cherry can balance the richness of the whiskey, making the drink more enjoyable.

While garnishes can indeed add visual appeal and help signify the drink type, the primary role in this context is focused on flavor enhancement, which is why this answer is recognized as the most fitting. Indicating alcohol content is not a function of garnishing but rather pertains to labeling and disclosure on beverages.