When one begins to turn the case mouth into the cannelure, the crimp ceases to be a taper crimp and becomes a roll crimp.
A taper crimp die can be handy if you want to use cast or jacketed bullets that don't have a cannelure or definite crimp groove, particularly with relatively light loads using these bullets. You can do this with a roll crimp die (set for "no crimp") but it just doesn't work as well. Otherwise, the roll crimp is the ticket with heavy loads using bullets that do have a cannelure or definite crimp groove. The extra die is kind of nice considering the $70-$90 cost of the set. I'd still discuss the process with the folks at Hornady and let them guide you.