Crimping as a separate step eliminates the pushing on the projectile while the crimp is tightening up around the bullet which crimping operation then tries to keep the bullet from moving.....while you are seating it........etc.etc.etc.
If the bullet is seated, the seating stem backed out, and then the case is crimped with a bullet sitting still you will have less tendency to bulge the neck or shoulder of the brass. Some of that bulging you may not even recognize without measuring the brass' neck just before the crimping is done. In other words, set up a die to seat and crimp in one operation, then back it out the thickness of a penny or nickle, then run the cartridge up into the die and back it out and measure the cartridge neck....then run the die down all the way into the press and run the ram up to seat and crimp in one step. See if you don't get a neck measurement that has increased after the second operation just below/behind the crimp......which can compromise accuracy to some degree.
I do some seating and crimping in one step....but not always, because of the tendency for some dies to cause neck or shoulder problems like I've described. I am not a fan of Lee products, but I do use their factory crimp-type dies.....seemingly with very good accuracy.
Good luck.