They provide two important functions. First, this is where all the movement comes from in your spine. The bones don’t change shape when you bend forward or to the side, instead your discs change shape and become ‘wedged’. This allows the vertebral bones and therefore the spine, to move in a wide variety of directions.
The second function is to absorb impact forces and provide stability. When you run or sit in a bumpy car-ride, you are constantly loading the spine with different ‘axial’ and shear forces. To help reduce some of those pressures, the disc can absorb them like a sponge.
The inner nucleus pulposus absorbs pressure forces, while the annulus fibrosus holds the bones and jelly-like nucleus pulposus in their places. However, when the disc becomes ‘worn-out’ or degenerative, those pressures are no longer easily absorbed and so they are then distributed to the surrounding joints and ligaments, resulting in increasing back pain and strain.