A full view tells a shopper what a product is; a detail shot tells them how it is made. The weave of a fabric, the stitching of a collar, the teeth of a zip or the setting of a stone cannot be judged at the size of a whole garment or a whole ring. A detail shot gives that part the whole frame.
Detail shots belong among the additional images of a listing, after the main image. They are the pictures in which small lettering and repeated parts are easiest to count, so they deserve a careful comparison with the product. A detail can only be as sharp as its source: a part that is blurred or hidden in the original photo cannot be shown faithfully.