Why do dentist's take x-rays?

X-rays show areas between the teeth that cannot be seen visually in the mouth. They also show the roots of teeth which helps when an extraction is required.
Most dentists take what is called "bite wing" x-rays every 18 months - 2 years to detect decay between the teeth.

"Periapical x-rays" are taken of a particular tooth or area in the mouth. These x-rays are taken to help diagnose specific problem teeth.