Maybe they are to uniformed members of the military. I've never once seen a civilian use, display, carry, or talk about a challenge coin. They typically get thrown in the back of a desk drawer and forgotten about. We literally have no use for them whatsoever. They don't even make good paperweights.

I have a stack of fourteen of them. Most of them I was given for spending thirty minutes giving some random officer a tour of the lab. They're not exactly hard to get.