Crowns are the restoration of severely damaged teeth.  The most common cases when crowns are installed are after endodontic treatment (canal treatment/nerve removal). 

For what purpose?

After endodontic treatment, the tooth tissues stop receiving nutrition from the nerve and become brittle and the walls may break off, which may lead
to tooth extraction.  As a rule, endodontics is performed when there is already a strong destruction of the tooth tissues, plus another part of the tooth tissues is removed during the creation of access to
root canals. Thus, there is less than half of the initial amount of healthy tissues. 

After the end of the procedures, a seal is installed and the tooth looks whole, and the patient thinks that the tooth is exactly as strong as it was originally. But this is a misconception. The seal only restores the missing tissues, but does not give the remaining tissues protection and does not prevent them from destruction.
And in addition, the filling material itself is designed for small seals, since its physical properties are not designed for large chewing loads. At its core, it is plastic. 
But ceramics are still stronger and more reliable, which is why it is used for large dental restorations.
Patients are often afraid to put crowns, because they have an association with those crowns that were put earlier, stamped gold or metal-ceramic. 

Now crowns look completely different, they are made entirely of ceramics or zirconium dioxide, and restore aesthetics identical to their own teeth (see photo)

And besides this, the most common fear is that the tooth is being ground. Yes, indeed, it is. But this is necessary in order to put the crown on top, and then the crown, like a shell, will protect the remnants of tooth tissue from chipping and
destruction. And in addition, it will be possible to fully use the tooth and chew.