Another common reason for fusion is a condition called spondylolisthesis. This is basically a slippeage where one vertebra slips or moves over the top of another. It is usually low down in the lurabar spine. Often a nerve becomes "pinched" when this happens. The spine also becomes unstable and they fuse for this reason.
Briefly a fusion is performed whenever the spine is deemed unstable. This could be the result of a previous laminectomy, discectomy, etc. or it could be the result of degenerative changes, congenital issues that are acting up, etc.
Any surgery should only be a last resort after all other conservative procedures have been tried. Having a back problem is not like any other condition in that you don't go to the doctor and have it fixed. Treatment is usually a long process and involves some trial and error. Keep in mind that surgery is not performed in an attempt to return the patient to the way he/she was prior to the injury or "hurt." Surgery is done mostly for relief of pain. If you're lucky, it can make you almost "good as new." Sometimes surgery takes care of one pain, but the patient enRAB up with new pain. There are NO GUARANTEES.