Question

imagine if our back bone was a single bone

Answer 1

we would not be able to bend

Answer 2

spinal cord is made of a long bone rather than a series of small bones (segments) then we will have difficulty in movements.

There will be problem in bending forward, backward and sideways with flexibility.  The shape of the back of human body is due to the shape of the spinal cord.  The stress withstanding is more with the segmental structure.