.357 mag. and .38 SPL jacketed bullets usually have a crimping groove in the bullet. Some lead bullets also have crimping grooves. The roll crimp die is for crimping into these grooves. The biggest benefit, is it keeps the bullet in place in the cylinder of a revolver, so the bullet doesn't jump out of the case in the cylinder during recoil. It also allows the slower burning powders for magnum loads to get a complete burn. Any decent brand of crimping die works well. I mostly use Dillon now, but have used Lyman, RCBS and Lee. When crimping separately, the crimping die is the last die used in the manufacturing process. Many dies come with a combination seating/ crimping die. I find it much more difficult to get them adjusted as a combination. Doing the crimp process separately allows me to get a much more precise crimp without pulling out my hair adjusting it. All of my auto loads get a taper crimp. It's really the only way to fly when loading ammo for most common auto calibers, i.e. .45 ACP, 9mm, .40 S&W, etc. You'll soon find out that loading lead bullets and coated lead bullets are a lot cheaper than copper jacketed or plated and you can get great accuracy for range shooting sessions. Many semi auto bullets don't have crimping grooves, so you must figure out just how deep you want to seat them in the case. You can sometimes find the Cartridge Overall Length (COAL) in a loading manual or listed by the manufacturer (you can call bullet manufacturers to get COAL data too! One important thing to remember is that they must be seated deeply enough to feed through your magazines. The taper crimp must be firm enough to keep the bullets from pushing back into the case. A bullet pushed back into the cartridge case will cause pressures to rise and if discharged can get to dangerous levels.Sound advice. What kind of dies do you use for roll crimp and taper crimp? what's the advantages of both?