Two advantages of using helical gears rather than spur gears in a transmission system are

Helical Gears have teeth that are set on an angle to the gear axis. Since the teeth engage more gradually, they have a smoother and quieter operation than spur gears. Helical gears also have greater tooth strength and a higher load carrying capacity. Further, helical gears can transmit power between either parallel or non-parallel shafts, while spur gears can only transfer power between parallel shafts. A downside to helical gears is they produce an axial thrust that needs to be accounted for; something not needed with spur gears.