A spoon full of sugar helps the medicine go down. A tumbler of gin helps the medicine go down, used to be more like it. For centuries, bitters was thought to have a medicinal value. And many people drink bitters after dinner as a digestiv — to aid digestion.
But for hundreds of years gin was by far the favored way to dilute their potent taste. As David Meredith Reese lamented in 1832 in New York, the drinkingest of drinky cities, decried the drink, and the combnination of Gin and Bitters as merely another excuse to pour more gin down the gullet.