Why is the understanding of dermatomes an important diagnostic tool?

Dermatomes are areas of the human body that are innervated by a specific area of your spinal cord. These areas are clearly defined, and can be tested.

The purpose of these? Let's say you had a burning sensation in the upper outer aspect of your thigh. If there is found to be NO PHYSICAL DAMAGE (ex. negative MRI, X-ray, ultrasound), the problem may be with the sensation that your leg feels. Because we know where it hurts, we can tell which dermatome it is in, which will lead us to the correct level of the spine in which the injury is beginning.