Arrgh ! I was wrong! Again !
Barefoot Friar was on the money. I was under the belief that the slanted rifling imparted spin as the slug flew through the air. Nope !
Bernneke's website has a nice explanation. Like Friar stated, the rifling is there so that the slug can safely pass through a wide range of normal chokes. It is the weight forward design of the slug that enhances the accuracy. I must add that out of my old spump 12 ga, 23" barrel, none of the rifed slugs I tried grouped well at all and the drop was much more than I expected. I did find an Outdoor Life You-Tube that showed a sabot out of a smooth bore ... tumbling along towards a target. Interesting.