It also saves pikes.
There's a page and a half in the welcome book about saving pikes. Apparently, predatory fish are necessary because they eat the small fish that feed on algae-eating crustaceans. With fewer algae-eating crustaceans you get more algae, which messes up the water supply or something.
Anyway, to redress this balance, the hotel has a "save a pike" campaign. If you hang the sign on your door, the hotel won't clean your room for that day and will send 20 SEK (about £2) to the Swedish Anglers and Fish Conservation Association for them to help save pikes (they operate pike factory breeding grounds).
The Swedish for "save a pike" is "rädda en gädda", which looks as if it rhymes. It must be true, then.
I've saved a pike every day so far except one, because the bin was getting full.
The sign has blown off the doorhandle and onto the floor on every occasion.