YETIRambler 12 oz. Colster Slim Can Insulator for the Slim Hard Seltzer Cans, Rescue Red (NO CAN INSERT)

what is the difference between no insert and can insert?

what is the can insert for?

Can I fit a 16 ounce can in this? I understand it'll stick out a bit and I don't care. Will be fine width-wise?

I bought one not too long ago for 12oz van and ended up being for 16oz can, is this only for the 12oz?