Like cars, teeth can break down too, especially after some fillings done, which means some of tooth structure has been replaced with some filling material like amalgam and composite resin.
In this case, we can see this upper second molar had a large amalgam restoration, two cracks on the buccal and distal aspects, and a fractured mesio-buccal cusp. During the first treatment, we removed all the old amalgam filling and replaced with a white filling. Because the tooth had lost too much tooth structure, it wouldn't be strong enough to stand normal chewing force and the chances to break again was really high.
The crown was the most appropriate treatment for this tooth in long term prognosis.