Fillings fit within the confines of the tooth. In contrast, a crown fits around the tooth and over its top. This difference may not seem all that important at first thought, but when considering the possiblity of tooth fracture, this is vital.
Teeth are weakened over their life-span by decay, recurrent decay, root canal accesss, loss of blood supply, excessively large fillings and heavy wear. Weakened teeth will usually, given enough time, split, break or fracture. These splits, breaks or fractures can be a death sentence for the tooth. When a fracture does occur, sometimes a crown can save that tooth--but in actuality, the proper thing to do is restore a compromised tooth BEFORE it breaks.
The tooth to the right, with a large filling and recurrent decay, has split into two halves. It is now unrestorable and must be extracted.
During examinations, your dentist will point out weak, split, cracked or decayed teeth which they believe to have a high possibility for damage. A crown may be suggested to protect these type of compromised teeth.
Unlike a car, which we can run until it breaks down and then fix it up with new parts. Often a tooth cannot be fixed and there are no "teeth parts" you can install when one is lost (at least not real teeth). As such, taking definitive, proper, preventive steps to keep splits, fractures or breaks from happening only makes sense.
Large fillings, like the one to the right, very often have thin portions of tooth on either side of the restorative material. This thin tooth structure is intrinsically weak and even moreso when all of the inner layer--the dentin--is removed from under the enamel. This is called unsupported enamel and it is usually just a matter of time before it breaks. If the break is too low on the tooth, it can mean expensive crown lengthening surgery to save it or it may even mean that its non-restorable and will need to be extracted.