What are bitters and why do cocktails use them?

The answer

Bitters are highly concentrated flavorings made by infusing botanicals such as herbs, spices, roots and peels. They are used in tiny amounts, often a few dashes, to add depth and balance to a drink, much like salt does in cooking. Aromatic bitters are common in old fashioneds and manhattans, while orange bitters suit martinis and several other classics. A small bottle lasts a long time, which makes bitters one of the best first purchases for a home bar.