Your dentist can see the surface of about 80% of the tooth and gums. They cannot see between your teeth where the flossing areas are, or under your gums to evaluate the bone levels for indicators of gum disease. X-rays are needed because that flossing area of the tooth usually accounts for 70-80% of all cavities we have. This shows the importance of cleaning this area by flossing and the importance of x-rays to check the area for disease.
Periodontal disease or gum disease is usually a silent process that has little to no painful indicators to let you know there is a problem. The main method a dentist can use to evaluate the problem is to combine visual cues and measurements of the gingiva and x-rays to show changes at the bone level. Without the x-rays we simply cannot diagnose or treat gum disease properly. If gum disease goes without detection and treatment, the end result is often the loss of bone around teeth and the loss of teeth.
The current guideline for the average patient is to get x-rays one time per year to adequately evaluate for cavities and gum disease.