The cracked tooth is the saddest reason for a root canal. This usually occurs because a tooth has a large metal filling in it. In most cases this filling was placed many years ago and since it isn’t hurting the patient has resisted the idea of taking it out and putting in something stronger like an onlay or crown. Why go through the expense or the aggravation for that?
But eventually, the large filling shifts minutely and one good strong bite down (a human molar bite is one thousand pounds per square inch), and the metal filling acts as a wedge and cracks the tooth. At that point the nerve becomes exposed and a lot more treatment than the original recommendation to change out the filling is now needed. We call that penny wise but pound foolish.
In dentistry we have seen teeth removed and over time the remaining teeth start to collapse in and the jaw is not the same as when the tooth is saved. The only thing holding the teeth in proper position are the other teeth. When one or more of them are removed the others collapse like a house of cards. Nobody can function properly with part of their mouth collapsed.
Doing the root canal can save the tooth structure because just the nerve is taken from the tooth. The root itself remains in the bone in its proper position. So after the nerve is removed, the canal the nerve was in is filed and smoothed and sterilized with medication. It is then sealed permanently with cement. The broken part of the tooth is then protected with a crown so the tooth can function properly for a lifetime as intended.