Assuming roll crimp is only used with revolvers, and can only be applied with revolver caliber reloading dies?
I'm gonna take a WAG, and guess you are asking about revolver calibers!
Roll crimp is used on revolver calibers for one or two reasons.
1. To keep bullets of any weight from being pulled out of the cases due to hard/heavy recoil.
That could apply to hot loaded 110 grain in an air-weight S&W .38 Spl snubby, or a 500 grain bullet in a S&W .500.
2. To increase bullet pull / case release to promote higher pressure initial powder burn when using slow burning Magnum class powders.
As an after-thought, a fairly heavy roll-crimp is sometimes used with light loads with little recoil to promote easier loading when using speed-loaders in a revolver.
rc