What flavor do tonic water primarily add to cocktails?

Tonic water primarily adds a bitter flavor to cocktails due to the presence of quinine, which is a compound derived from the bark of the cinchona tree. This bitterness is a distinguishing characteristic of tonic water, setting it apart from other mixers like soda or juice, which may introduce sweetness or sourness. The bitterness of tonic water is often balanced by the other ingredients in a cocktail, pairing well with spirits like gin, where the complex flavors enhance the overall taste profile of the drink. This contrast adds depth and interest, making it a popular choice in many classic cocktails.