Functional crowns are cemented on teeth that have a hairline crack, a super large filling that makes cracks likely, or several small fillings that are failing. The risk with many cracks is that they go any direction they want, including straight through the root of the tooth, and these cracks can get deeper or longer if the tooth is used for chewing. Crowns are also placed on teeth that have had a root canal because they become more fragile when their blood supply is removed.
In contrast, esthetic crowns are cemented on teeth to improve your appearance. Perhaps a tooth chipped. We also see teeth darken with age, old fillings get stained, or years of chewing have worn them flat and functionless.
Most posterior crowns have a substrate of cast metal, or gold, for strength. Tooth-colored material is baked on so the crown matches your other teeth, in many situations. It is cemented into place. The only part of the tooth that isn’t covered is the root surface, so it might still get a cavity if you don’t watch your sugar intake, brush well with fluoride toothpaste, and use floss.
This risk is far less than the problems created if you just removed the tooth and left surrounding teeth to drift into this space.
To remove a crown that has developed a cavity that we cannot reach, we usually drill it into sections and then clean off the residual cement. Once in a while, we are able to use a counterweight apparatus to tap the crown until it loosens. If it comes off in one piece, without damage, it can be re-cemented.