Over time teeth wear and chip. The edges of front teeth have a protective covering of a little more than one small millimeter (1/25 of an inch) of hard enamel. This enamel can chip and wear creating ragged and sharp edged teeth.
Once the initial edge enamel is worn off, the enamel on the front and back of a tooth loses its major protection and more easily chips away. Using fine diamond drills and thin sand paper abrasives the edges can be smoothed, corners reshaped and polished to resemble their original, or sometimes better than original appearance.