As said the crimp helps to burn the powder more efficently. Especially with the slower burning magnum powders.
Also the crimp helps keep the other rds in the cylinder of the revolvers, as to the bullet heads pulling out from the recoil of the one that you fired.
Much like a bullet puller works. I have an intresting story or expierment to tell.
I was loading 38 rds for my snub nose revolver, i ran out of bullets. I noticed that i had some 9mm bullet heads around. I didnt think this would work, but i loaded about a dozen 38 shells with them. I put a stong crimp on them to see if when one is fired, the others will not get longer(OAL)
To my surprise the other rds, did not get longer from the recoil of the fired one. I do not think this is a good way to load, and i do not recomend it. you always should use the proper DIA bullet head.
At first i never use to crimp my reloads, until one time i was shooting a bunch of reloads i had made that was toward +P in pressure. I wanted a powerful rd, i planned this way.
What i did not expect was after the 1st shot, that did have a nice kick to it, Would not cycle, i thought the rd was to hot and i had broken something. After close inspection of the other rds in the chambers i noticed the next rd that was in the chamber was way to long,not letting the chamber cycle.
With a small screw driver i was able to push the head back into the shell enoughto unlock the cylinder. Opening up and retreving the other rds. And that one rd was way to long.
So i found out the hard way, how much a strong crimp can help.
I think if the reload is made on the mild side, this may not happen. Because the recoil factor is not as great. But since this incodent i made it a habit in buying the extra crimp die. Especially when reloading for +P or magnum loads.
I also use the exrtra crimp dies in making my auto rds.I find it takes that sharp edge away fom where the bullet head meets the top of the shell, and allows the bullet to feed up the ramp easier. And if you want to use a slower burning type of powder, you get a more uniform burn. With autos i tend to stick to the faster powders usally, but i figure it cant hurt.
I buy lee carbide dies and the 4 die sets are really not that much more.Generally you want a roll crimp for revolver loads and a taper crimp for the autos.These dies are already figured for the right type crimps from the factory when you purchase them,In other words when buying the sets or adding the crimp die to your set, Lee is not going to sell you a roll crimp die for say a .45 auto.
I know you can crimp with the third seating die with out buying the seperate crimp die,but than you have the concerns of if you are crimping to much or using the right crimp etc... It can get confusing. Lee advertises that there is no way you can over crimp with thier factory crimp die. If you turn the threads in to much it wont ruin the case. I found this to be true also.They take the mystery out of the process, and lets face it we have enough to think about as it is.
Hopefully this helps and i am sorry about the rambling on. Bad habit of mine. Take care and happy loading. Jack