Traditionally, if your tooth had a cavity or fractured you had a filling done to repair it. Normally the tooth would be repaired with silver amalgam, gold or more recently, tooth coloured plastic. For small or conservative repairs these materials worked fine and for decades but for replacing large sections of the tooth their physical properties often led to the premature breakdown and sometimes loss of the tooth. If lucky, you can have a porcelain and gold or pure porcelain crown covering your tooth and regaining strength. The only problem is that it usually takes two appointments to complete, one for preparation and shaping of the tooth with taking moulds for a lab technician to make the crown. Two weeks later you have to return to have the temporary crown removed and the permanent one cemented in place.
Cerec gives you the same strong tooth at the same cost but in only one appointment.
A good site to visit is that of the manufacturer of the equipment. They are obviously a little biased but their claims are all quite accurate as you explore the site. http://www.cereconline.com/