I was going to say liquor, but yeah. You can use it for soda too if you buy a 12 pack and bring it back to the hotel with you instead of letting the drink machine nickel and dime you.
Yep, champagne is our main use case. If the wife and I are staying someplace nice, we love to get a bottle of champagne and some nice cheese at a local store and hang out in the room at least one night.
The last hotel I stayed at (fancy expensive hotel for a company gathering) had a mini fridge stocked with ridiculously expensive items, in such a way that the fridge was unusable for outside items. There was also a note that any items removed from the fridge would automatically be charged to the room. There was one bottle of complimentary water on the counter though.
God there was one where I bumped against the fridge and shifted a bunch of items that all showed up on my bill. I think a lot of the Disney hotels work on that system as well.