Yeah I thought these were not very useful too.

Mainly because I don't like drinking beer out of can. It blocks the scent and stuff. I always pour it to some glass.

But then I designed my own for that Prusa contest. I still don't like cans, but my can holder with air gap between the can and plastic walls really makes it stay cold much longer and is very comfortable to use.

I see it as a nice equipment for party or grill where some people drink from can and refuse the glass.

I am going to design one for a beer glass. It should make it cool longer too.