High-speed handpieces are primarily used for which of the following tasks?

High-speed handpieces are primarily designed for tasks that require precision and efficiency, particularly those involving the cutting and shaping of hard dental tissues. Refining a cavity preparation is a key task where high-speed handpieces excel because they allow for quick removal of dentin and enamel while maintaining control over the preparation's shape and depth. This is essential for ensuring that the cavity is appropriately shaped to receive a filling material, promoting better adhesion and longevity of the restoration.

While high-speed handpieces can indeed be used in the removal of old filling materials, they are more effective in tasks that involve cutting through harder substances. The primary advantage of high-speed handpieces lies in their ability to perform cutting actions with speed and precision, which is particularly vital during the cavity preparation phase rather than during filling removal. Thus, refining a cavity preparation is the most accurate representation of their primary function in dental practices.