Onlays are sometimes indicated in these following circumstances;
An onlay is considered more of a long term restorative option for a broken down tooth rather than a conventional white filling. This is because conventional fillings may need repair and replacement over time due to wear and tear. This is especially so on back teeth (molars) and in cases where someone has lost some teeth so the molars are working harder to chew.
Similar to dental crowns, onlays are ideal to strengthen and support the remaining tooth structure. They are the superior option over direct dental fillings to protect weak teeth and prevent cracked teeth from breaking further. They have more predictable long term strength and they preserve more tooth structure to avoid the need for repeated repair and maintenance.