Three reasons for check-ups: periodontal disease, tooth decay, and gum disease.
The first reason is that the body naturally builds up plaque and calculus and if it's not removed it embeds underneath the gum tissues and causes periodontal disease. Dentists call periodontal disease the silent killer. The disease doesn't hurt but it silently produces enzymes that dissolve away the bones. That's exactly how people lose teeth.
The second reason is tooth decay. A little cavity can be taken care of. A big cavity becomes a compound problem. In its largest stage, it can cause suffering and swelling but also the loss of a tooth. We don't want to lose teeth because that's the main way of chewing food and our main support system. It starts a domino effect by putting more pressure on the teeth and causes more compounded problems. Lost teeth don't only make it more difficult to enjoy food, but also cause digestive problems.
The third reason is that there's a strong correlation between gum disease and heart disease. Plaque on the teeth produces billions of bacteria that ends up in the blood stream. While bacteria normally exists in the mouth, gum disease increases that level so dramatically increased that it gets carried through the blood and can end up lodged in the heart and clog blood vessels.