They are necessary for a variety of reasons:
For decay detection
During an examination, decay in a tooth can be seen if it is in an area that is visible in the mouth. Often, however, decay and other pathology can be situated in between teeth or in areas where it cannot be easily visualised with the naked eye and hence remain undetected. This is where radiographs are necessary. The earlier a problem is detected, the simpler the treatment required. There are different types of radiographs taken for different purposes and they are recommended as appropriate.
For root canal treatment
During root canal treatment, radiographs are an absolute necessity as the dentist is operating inside the tooth within the root system. This is not visible as the roots of the teeth are beneath the gum line and enclosed within the jaw bone. Radiographs are needed to give an image of the roots structure, length and shape so they can be cleaned and filled.
For pre-assessement before major dental treatment:
Prior to starting orthodontic treatment, radiographs are required, firstly, to ensure there is no decay in the teeth, and secondly, to visualise the position of the teeth in the jaws.
Prior to placing an implant in the jaw, a radiograph of the area is necessary to ensure that there is enough bone thickness. The proximity of anatomical structures, like the sinus in the upper jaw or the inferior alveolar nerve in the lower jaw, need to be determined so the implant is placed in such a way as not to impinge on these structures.
Crown and bridge work
Prior to placing a crown or bridge, the teeth being crowned need to be assessed to make sure they are healthy and strong enough.
Prior to extraction of teeth
Radiographs show the hidden root structure of a tooth and also its position relative to other teeth and other anatomical structures. This is helpful to plan the extraction to minimise trauma during the procedure and is especially important when wisdom teeth are being extracted
For general diagnosis of pathology and gum disease
Pain in the oro-facial region is often dentally related. Radiographs can be helpful in confirming the diagnosis. Simple visual examination might not provide enough information to allow the dentist to reach a definite diagnosis.
Radiographs also allow assessment of gum disease severity.