Because they dry and stiffen much faster. When the handles are glued on with slip about an hour later they are about the same stiffness as the mugs. The handled mugs, which are sitting on plaster batts, will then be covered with cloth and plastic over night. The next morning, the bases will have stiffened and all sections of the mugs will be about the same stiffness, ready for trimming. After that they will be decorated, then placed on smooth wooden batts and wrapped with a cloth for drying.
These mugs are in the leather-hard stage. They have just been trimmed. The clay has dried firm enough to handle them without pushing them out of shape too much, but not so much that it is difficult to tool and flute the foot ring.