Full metal crown for badly carious teeth

I was studying the topic of full metal crowns (fixed prosthodontics), and in my book it says that one of the indications of making a full metal crown is that the tooth that is going to receive the crown is badly damaged by caries, or that the oral hygiene is bad in general.

I don't seem to get the relation between full metal crown preparation and the damage of teeth by caries.

Any ideas?