I like to pull nmwu into a raceway:
1) because I can run the raceway and pull the wire into it after the hot-tub is installed, to avoid damage to the cable,
2) to provide an extra of protection, especially where the cable comes out of the ground, but also where it crosses other underground systems, or where rodents are a problem,
3) to allow the wire to be withdrawn and re-pulled without trenching,
4) where I would normally install conductors but because of the location a junction box or flex are impracticable at the ends of the raceway, or
5) where others might install a water pipe, because I have easy access to pvc but not water pipe.
