Ceramic onlays are the best solution to restore teeth with over-extended caries or previous over-extended old failing restorations. Sometimes a simple new composite filling might not be the best restorative solution because it doesn’t reinforce the tooth as a ceramic onlay is capable of.
Also, with the admin of modern dentistry, I avoid doing crowns as much as possible, I rather aim to preserve as much tooth structure as possible. This approach is also considerably reducing the need for root canal treatments and many studies are proving it is prolonging the life of the tooth.