There are a number of reasons the use cream of tartar in cookies is important. It acts as a stabilizer, helps prevent crystallization, and provides the needed acidity to a recipe without the use of additional liquid. Though a number of other acids are just as effective, cream of tartar is used instead because it is not a liquid, which can further alter a recipe. Using this ingredient helps yield a cookie that has a pleasant texture and taste.
The fact that it acts as a stabilizer in egg whites is one of the main reasons individuals use cream of tartar in cookies. This keeps the whites fluffy and stiff, which means they are able to stand up to the addition of ingredients, mixing, and baking at high heat. It also means the dough remains fluffier, thereby creating a light and airy cookie that rises while baking. In some cases, cookies made without it appear thin and flat with a dense texture.
When sugar crystallizes in baked goods it creates an unpleasant and gritty texture. Cream of tartar in cookies helps prevent this undesirable condition by keeping the sugar from crystallizing. It is also used in creamy products such as frosting to ensure a smooth texture.
Since a base needs an acid to create a reaction, cream of tartar in cookies provides the acidity needed to react with the baking soda base. This causes the reaction necessary for leavening of the cookies. Most cookie recipes involve first creaming butter and sugar together, which causes air bubbles that might then be expanded during baking. To aid in this process, an acid and a base are added to help the air bubbles expand while baking, usually creating a better texture within the finished product.
Cream of tartar in cookies is often used because it has no odor or taste. Other acids such as vinegar might be used, but adding a liquid would likely alter the properties of the recipe too much. This would probably yield a product that is undesirable in both taste and texture. In addition, the use of further liquids in the recipe would alter the dough and likely interfere with baking.
A variety of other recipes use cream of tartar as well. Sometimes it is used for similar reasons to those above, and other times it used for slightly different purposes. It is sometimes used for crafts or for other non-edible purposes.