Because the gravity of our earth, ordinary Chairs are just not able to support the downward pressure directed at the lower lumbar area of the back.
Even if you are sitting in an ordinary chair, and the weight is off your feet, you spine is stressed all the time.
The weight of the body compresses the spinal discs, those pillow like cushions that sit between the vertebrae of the spine.
To stay healthy discs need to be hydrated otherwise they lose their flexibility and their cushioning effect declines.
If you think of the back as being shaped like an upside down triangle, with the top part of the triangle being heavier than the bottom, you can see how pressure is focused downwards toward the lower back.
Poor seating can lead eventually to all sorts of serious back problems not to mention stress in the paraspinal muscles on either side of the spine.
