Spinal fusion grafts the two vertebral bones together so they don't move. Motion is lost but the segment is stabilized. This helps reduce pain. The main problem with fusion is degeneration at the next level (either the level above or the level below).
On the other hand, lumbar disc replacement (LDR) preserves motion. The idea is to prevent adjacent level degeneration and prevent future recurrence of pain. Often a fusion at one level requires a fusion later at another level. The LDR may prevent that from happening.
We say "may" because long-term results aren't available yet. Some studies have data up to eight years. There are no studies yet that follow patients for the life of the implant (15 years).