Keeps motion. The benefits of successful disk replacement include preservation of motion of the spine, no stress on the adjacement disk spaces, and avoidance of fusion. They are not for every patient and there are some pretty strict guidelines as to when they are useful. See a surgeon who does them often with great experience in this surgery.
The same as a fusion. If you have been indicated for a total disc replacement, the results have been shown to be equal to that of a spinal fusion in the properly indicated patient. The goal is pain relief in both procedures with the theory being that by maintaining motion, you may reduce the chance of an adjacent disc level from going bad. This has not been proven to date.