Lots of possible answers sums it up pretty well. For the .45-70, there is nothing wrong with the LFCD at all... Especially if you are loading large diameter for caliber cast bullets and failed to adequately chamfer the case - seating in one step then crimping in a second can possibly save some cases from damage. The .308 loaded with jacketed bullets - I stick with the integral crimp built into the RCBS seating die, but I still do it in a separate step after seating to the desired length. For the 9mm, which requires a taper crimp since it headspaces on the case mouth anyhow, there is no benefit to a separate die, IMO, but I still crimp as part of a separate step following seating the bullet.
Matter of fact, I treat crimping as a separate step in all my loading work - - After all these years, it is a habit that works well for me, and IMO, it is just part of the deal.