What is the purpose of small amounts of bitters in cocktails? For example, 1/32nd tsp angostura bitters in a Manhattan? Do they provide any function other than flavor, such as binding, texture, or mysterious chemical reaction?
Bitters originally were supposed to have medicinal properties, which is why some still have "tonic" in their names. Now they're just a flavoring agent, a concentrated way to enrich the cocktail and bring out more subtle flavor notes.
Given how little bitters you put in most drinks, it's unlikely they alter mouthfeel or other qualities much.