What is the common use of a "zester" in the kitchen?

A zester is primarily designed to remove the zest of citrus fruits, which is the outermost layer of their skin. This outer layer contains essential oils that are extremely flavorful and aromatic, enhancing the taste of various dishes. When zesting, the tool allows for the extraction of this flavorful skin without the bitter white pith underneath, which is often undesirable in cooking.

The other options, while related to kitchen tasks, describe functions that are better suited to other tools. Peeling vegetables generally requires a peeler, which is specifically designed to remove skin without taking off too much flesh. Extracting juice from fruits is typically done with a juicer or a citrus reamer. Shredding cheese is more effectively accomplished with a grater or a box grater. Each of these tools is optimized for their specific functions, making the zester uniquely valuable for incorporating citrus flavor through zest.