As you can see in the photo on the left, the tooth has fractured below the gum line.  If  your dentist were to try to restore the tooth with a crown,  the margin of the crown may be on unsound tooth structure or on the remaining filling (which may be very weak at best.

The photo at the right shows the result of the surgical CROWN-LENGTHENING procedure done by your Periodontist.  It shows that the gum has been moved down the tooth by about 3-4 millimeters.  Now your general dentist has enough sound tooth structure on which to end the margin of  the crown.

As you can see from the photo on your right,  the final crown has been placed with a solid metal (gold) margin to properly seal the crown to the tooth.  Although there is a significant cost to perform the procedure,  you can be assured that you won't be back to your general dentist a couple years from now to redo the crown because of recurrent decay from a leaky margin. Infact, quite often times, the recurrent decay is not caught in time and a root canal may also be necessary.