Final answer:
Old sherry casks are used for maturing whisky to impart unique flavors from the absorbed sherry qualities, whereas new casks may overwhelm the whisky's subtle flavors. This practice is also traditional and sustainable.
Step-by-step explanation:
Old sherry casks are used for maturing whisky rather than new ones due to the complex process of flavor development that occurs during the whisky maturation process. The use of old sherry casks adds unique flavors and characteristics to the whisky, as these casks have absorbed the qualities of the sherry they once held. These flavors can include dried fruits, nuts, and spices, which contribute to the whisky’s final taste profile. In contrast, new casks might impart too much tannin and oak flavor too quickly, potentially overwhelming the subtle flavors whisky makers seek. Additionally, the practice of re-using casks is a traditional and sustainable approach, and it helps to create a consistent product over time.