So you can see what burrs on the inside of the case mouth will do...
What's different on the outside? That cartridge case has to sit in a nice, tight, form-fitting chamber, right?
It just makes sense. Like making a clean cut in a piece of wood, sanding it and then blowing the sawdust out of it before you'd ever think about staining the wood, ya know?
Attention Brass rats and other reloaders: I really need .327 Federal Magnum brass, no lot size too small. Tell me what caliber you need and I'll see what I have to swap. PM me and we'll discuss.