The purpose of dermatomes is to show where a problem may lie within the spine. For example, L3 (Lumbar Vertebrae 3) dermatome area is to the bottom part of your upper thigh and your knee. If you experience a problem here in your upper thigh, it gives clinicians an idea that L3 may be causing a nerve problem.
There are also myotomes. These are nerves that supply a muscle. For example, the myotome for L3 is the muscles that produce knee extension. If the patient cannot resist knee extension, this may suggest again a problem with the nerve root at L3.
Pins and needles and numbness are also signs of a nerve problem. Along with sharp shooting pains, and burning sensations/pain.