A comprehensive dental exam goes far beyond looking for signs of tooth decay. It's also necessary to take a complete medical history to alert the dentist to health conditions and/or medications that may affect future oral health and dental treatments. The dentist or hygienist will want to assess the health of the gums as well, by measuring the depth of periodontal pockets, plaque/calculus buildup, and inflammation. This is important because gum disease can lead to tooth and bone loss and has been linked to heart disease, diabetes, and other diseases. In addition, it's important to look for signs of mouth and throat cancer, as well as other suspicious growths and cysts that a dentist may be first to spot.
P.S. A comprehensive dental exam includes assessments of tooth positioning; tooth loss; signs of tooth grinding and jaw clenching (which can lead to headaches, tooth erosion, and TMJ disorders); and signs of how previous dental work is holding up.