Why would a mri of the back be ordered without, and with contrast? what is the contrast enhancing?

It depends: It depends what they are looking for. It is common to perform a spine MRI without/with contrast. If you had prior back surgery, it will help differentiate post surgical scarring(which enhances) from other processes like disc extrusion. Inflammatory and malignant abnormalities will also generally enhance. Vascular abnormalities will enhance.