If your backbone were just one long bone, you would not be able to move side to side or bend of twist your back. Your back would be stiff.
It would be difficult to sit down
each vertebrae acts as it's own little stretching tool.
They all work together so that you can be more flexible.
With just one bone, you would always be completely straight up like a robot.
A flexible spine is what helps people (and animals) move around freely.