The dental literature has proven that placing a protective cover or crown over weakened tooth structure will give it longevity and prevent tooth loss.
The new crown materials that we have the luxury of using nowadays, have the benefit of strength AND beauty. No longer do we have to compromise esthetics for strength.
Once all decay is removed and a build up material has replaced the defective tooth structure (think of it as pouring foundation), we can proceed to preparing the tooth for a crown. Basically, this entails removing the outer 2-3 mm of tooth to make room for the crown, or cover, over the healthy remaining tooth structure.
The beauty of modern crown materials is, not only are they beautiful and strong, but we now have the ability to remove minimal tooth structure to make room for a crown. This is because many crowns are now bonded in place rather than cemented. This means the use of chemical reactions to actually fuse the crown to the tooth thus requiring less tooth reduction. Whereas we used to rely strictly on tooth reduction to mechanically keep a crown in place, we now are using cutting edge science to bond a crown in place.