Using flavors available in 1800's America, Fee Brothers developed Rhubarb Bitters for an authentic historical taste.
The effect of Bitters in a drink is to balance, round out and complex the flavor. Your mouth has taste receptors for sour, sweet, and salty; AND it has a receptor for bitter. The more receptors triggered as a drink tickles on the way down, the more interesting the drink. You know you have used Bitters correctly when the person enjoying the drink says, "Wow, there is something going on in the background of that drink". Consider that a slam-dunk.