Crimping is probably of no value for ammo used in slow-fire bench-rest or target work with bolt-action rifles.

HOWEVER, in semis, (or full-autos), it is a "good thing", because the ammo gets shoved from the magazine quite violently. Some rifles are "nicer' than others. An M1icon rifle will, if in good condition, feed empty cases from the clip, straight into the chamber, without damaging the case neck. Other designs? Not so much. Furthermore, if your ammo is sloshing back and forth in the magazine with the recoil of every shot, un-crimped projectiles MAY start to move, this altering all your good work at the loading bench.

I started using the Lee "Factory Crimp" dies when they first appeared, for these reasons. Note that pretty much ALL "issue" ammo is crimped, usually at the end of the neck.

What I found interesting was that the biggest performance improvement came in a couple of "long-range" loads for .223Rem. One of these involves a compressed powder load that was showing odd variations in muzzle velocity and grouping. A delicate application of the Lee die successfully ironed that out. YMMV, and all that.

The Lee dies can be set to "just" form a crimp or form a deep crimp that will impress a substantial cannelure into a plain, jacketed bullet. In the dark ages, I had used a hand-operated cannelure-forming tool; slow and painful.

If you look at original .303 Mk7 ammo, it does NOT have a mouth crimp, but radially-impressed "stab"-crimps back along the neck. One of my shooting buddies has 're-engineered" a Lee die to replicate that style of crimps. I'll await the results.

The only problem is that every time the mouth is "folded" in to form the crimp, it works the brass pretty hard. Even with regular annealing, (and trimming!), splits will start to occur. Brass cases are ultimately a "consumable" (but recyclable) item. If you get more than ten reloads out of a .223 firing decent loads from an AR platform, or .303 from a Lee Enfield, you are doing well.