Stability stops pain. The spine is made up of many moving parts, joints, ligaments, bones and disks. When the parts become damaged, unstable or worn, movement becomes painful. A spinal fusion takes the moving segments and makes them stiff by growing bone across them. Once the movement stops, it's no longer painful. You will lose some mobility and it is possible to develope problems at adjacent spinal levels.
Varies. For unstablefractures, it restores stability allowing mobility & protection from any potential further neural injury. For deformity like scoliosis, it can correct it & prevent further progression. For tumors & infections, it allows these to be treated surgically along with medical treatment and restore function/stability to spine. For back pain, it is less predictable but may it may benifit.