For many reasons. Sometimes people aren't able to maintain the recommended frequency of dental checkups. Oftentimes, when an extended time has gone in between cleanings, a more advanced cleaning may be recommended by your doctor. Harmful bacteria and hard tartar deposits called calculus build up on your teeth above and below the gumline, releasing toxins that cause early symptoms of gingivitis inflammation and bleeding in the gums. Unfortunately, this buildup occurs in areas that can't be taken care of By brushing, flossing, or even irregular dental cleaning, this buildup can become so extensive that a dental exam is not possible. If this is caught before the full onset of periodontal disease, your doctor can recommend a procedure called debridement as a first step to remove these deposits from your teeth. In some cases, this will be enough on its own to bring your gums back to health. Debridement is a specialized procedure that removes excessive calculus and bacterial deposits beneath the gumline, and is usually performed with a high tech device called an ultrasonic cleaner that uses high frequency vibration to separate the calculus from the tooth. By removing the deposits and bacteria, debridement is in many cases, able to help the spread of gingivitis. Your doctor will help you determine the next steps. Now that a comprehensive examination is possible, if these symptoms are not treated appropriately when detected, the progression can advance to the point of causing periodontitis and bone loss, which may require far more invasive and costly procedures to treat over time.