First, the normal 2 die set that most reloaders buy/use, does not crimp during bullet seating. Unless you are using a tubular magazine, no crimping is really necessary. Crimping serves several purposes, first it seals the case to the bullet, especially in pistol straight wall cases (and some straight wall rifle cases like 45-70). It also allows more pressure to build up, which helps seal the case wall with the chamber preventing blowback. It also prevents the bullet from being pushed down into the case when loaded in a tubular magazine.
I don't crimp at all for any rifle loads, only straight walled pistol loads.