Why do shotgun shells have brass or steel encasing the base while

Why are shotgun shells all rimmed? - Quora

Cleaning 10 Gauge Brass Shotgun Shells Presented by Larry Potterfield

I've seen short gunshot shells or mini shells for the first time last week. I'm a reloader and I reload shotgun shells. Seems like I can crank out a bunch of these;

What You Should Know About Shotgun Shells - Bird Hunting Article

Ammunition Selection Stock & Barrel Gun Club

How do shotgun shells work? - Quora

Do all 12 gauge shotguns take the same shells? - Quora

I got my grandpa Winchester model 25 12 gauge pump action shotgun and I'm trying to figure out if it should be able to fire steel birdshot or do I need to

Remington 9mm Ammunition LeadLess LL9MM2 124 Grain Flat Nose Enclosed Base 50 rounds

What is the diameter of a 12 gauge shotgun shell right before the rim? - Quora

Base Wad Separation