Same reason, I suppose, that RCBS .400 Cor-Bon dies also roll-crimp. I can't speak for the philosophy of the guys at RCBS; was just passing along information.
Some of us, btw, own cannelure tools and do, in fact, cannelure bullets that we use for bottleneck handgun ammo.
When .357 Sig was first developed, somebody put out the erroneous information that it headspaced on the case mouth. It headspaces on the shoulder, like other bottlenecks. Of course, if the shoulder's set back too far, it'll headspace on the case mouth. When you're setting up your sizing die, you're setting it up to bump the shoulder.