Taper "crimp" relies on neck tension for bullet retention, and to me is nothing but a bell removal die. Roll crimp actually bites into the bullet adding resistance to bullet movement along with neck tension, and definitely what I would want if shooting a revolver.
A bullet without a crimp groove was not designed to have a crimp "bite in to it".
The taper crimp is DESIGNED for semi auto pistol cartridges that are supposed to headspace on the case mouth l(even if the extractor prevents it from doing so).
The purpose is to restore the case walls so they're straight. Check out the dimensions of a factory round, virtually all of which are taper crimped, and then use your taper crimp die to duplicate.
Walkalongs' post accurately describes the proper use of the taper crimp.
Without a crimping groove (cannelure), the roll crimp does little more than deform the bullet.
The roll crimp works well for cannelured revolver bullets, or slight crimp over a wadcutter.
Semi-auto rounds still work, in spite of it, because the extracter prevents the improper roll crimp from allowing the case to travel too far forward, which could otherwise be quite problematic if the bullet didn't engage the rifling soon enough to still allow the firing pin to hit the primer.