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, the decay between teeth or in areas where it cannot be easily visualised with the naked eye and hence remain undetected. The following images illustrate how taking a radiograph allows detection of decay otherwise not visible to the naked eye.
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.
For pre-assessement before major dental treatment such as:
Crown and bridge work
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.
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.