Acidity provides the crisp, slightly tart taste of wine on your palate. Too little acid, and your wine tastes bland and non-committal and will be more susceptible to spoilage. Too much acid, and your taste buds scream for relief. When acids are properly countered by the other ingredients in wine (such as alcohol, sugars, trace minerals, etc.), the wine is said to be "in balance", which is the desired end result of all winemakers.
Use to increase the acidity level in your wine must.