The discs of the spine receive nourishment through blood vessels until approximately the age of 20. At this point, the disc has been filled with a lifetime supply of nourishment. However the disc has to make this supply of nourishment last a lifetime. Through aging and normal wear and tear , we use the nourishment and all experience a slow degeneration of the discs of the spine.
This is normal aging. The more "stress" you put on the disc, the more you use the nourishment, weakening and deteriorating the disc, creating a problem. The theory behind decompression is that it gives the body a chance to heighten disc space, in an effort to return to normal size and function.