To get the powder to burn correctly in rounds like the .45/70 and .458 Win., 450 Watts, crimping is about necessary. Too - it prevents both length collapsing due to recoil hammering the rounds into a magazine, or lengthening due to the same forces. At times, 'jumping the crimp' also happens when crimps are not sufficiently heavy.

Lee Factory Crimp Dies help in this regard and allow heavier crimps than normal roll crimpers in seater dies.

In bottle necked rounds, I only crimp .218Bee and .22 Hornet - again, to get the powder I use to burn correctly.

The .356 Winchester M94 needed crimping to maintain ctg. length in the tubular magazine.

Normal hunting bottlenecked rounds in bolt actions do not need crimping in my opinion, nor have they ever needed it. Proper neck tension is more than sufficient to hold the bullets where they are. Roll crimping can actually soften/loosen the hold of the neck on the bullet.