When root canal therapy is complete, the tooth is considerably weaker. Firstly the middle of the tooth has been drilled deeply, which means there is greater leverage on the cusps. Biting forces can cause the tooth to crack in half. Secondly there is no reflex reaction within the tooth when something excessively hard is encountered. Without a nerve to feel the crack starting, the tooth can shatter unexpectedly.
Crowns can help prevent the shattering, splitting and breaking of teeth after root canal therapy by bracing the tooth. Indeed in the preparation of a crown, more tooth needs to be removed to make space for the crown. However a skilful dentist will know how to minimise the cutting to give the optimum fit. For example using gold allows the dentist to cut less tooth and keep the tooth stronger.