They are very thin and are held in place by a special strong bond (rather like super-glue), so very little preparation of the tooth is needed. Some types of veneers don't need any preparation at all. A precise shade of porcelain can be chosen to give the right colour to improve a single discoloured tooth or to lighten the front teeth.
A veneer can also make a chipped tooth look intact again. The porcelain covers the whole of the front of the tooth with a thicker section replacing the broken part. Veneers can also be used to close small gaps, when orthodontics (braces) are not suitable or if a tooth is slightly out of position, a veneer can be fitted to bring it into line with the others.