The Spine Model is based on a chain of levels, and when written out, resembles the vertebrae of a spine - the central frame off which your body is supported.
We see a lot of crooked spines in organisations. Quasimodo-like creations that are doomed to failure. Crooked spines are caused by people only focusing on part of the Spine. Stay with Practices and Tools and implicit and misaligned assumptions are generated about why we are here and where we are going. Stay with Needs and Values and pain is felt when it comes to where, when and how we are going to accomplish anything.
If you want a straight spine, you have to start at the top, at Needs and work down, iteratively. This will keep your spine straight.
There are other ways to extend the spine metaphor, but this was the central thinking when we decided on the name (the model preceded the name).