There are four natural curves in the spine. We usually speak in terms of the 3 that comprise the cervical, thoracic, and lumbar portions of the spine; but as you can see, the sacrum and coccyx form a curved section as well.
The spinal curves provide architectural strength and support of the spine. They distribute the vertical pressure on the spine, and balance the weight of the body. If the spine were absolutely straight, it would be more likely to buckle under the pressure of the weight of the body.
When the all the natural curves of the spine are present, the spine is a neutral position. This is its strongest position and usually the safest to exercise in. When we have perfect posture the curves of the spine are helping us balance. We are meant to walk and stand in the neutral spine position. In Pilates, we work a lot with developing the musculature in a way that supports the neutral spine position.