Helps to control the acidity of your wine. Since it is the primary acid component found in grapes, winemakers can adjust the acidity of their wine by adding tartaric acid to the wine.
Tartaric acid is the characteristic acid of grapes which is found in no other common fruit. Low acid grapes from warmer climates will benefit from its addition; the wine will clear more readily and will keep and taste better. It buffers to a nice low pH. Wines with acidity below 0.5% will benefit from its addition. It takes about 3.7 grams per gallon to increase acidity by 0.1%. Best practice is to add tartaric very early in the process.