According to Catholic doctrine, why did Jesus Christ have to die to save mankind if only the physical body is involved in death?

Catholics say:The soul is immortal; all humans have one. Adam sinned and his physical body returned to dust, but his soul moved on to phase 2. According to Catholic doctrine, why does the whole plan of redemption depend on the physical death of Jesus Christs human body? How are we saved from Adamic death which according to doctrine was death of physical body only?