Toilets.
OK ok stop, not in that way.
I mean I like going through an area in a dungeon or castle and finding a room that has no purpose for you, or no reason gameplay-wise, or story-wise, but it just makes sense for it to be there. Going through a dungeon a band of goblins claimed for themselves, and stumbling upon a pair of out-of-the-way alcoves, each with a bucket and a puddle of something, and maybe a scrap of paper sitting nearby. No loot, nothing you can take (except the bucket and paper, which you can't even sell), it's just... there.
Same thing with other rooms. Walk into a room or a house, and inside instead of finding gold and a sword, all you find is a candle and a couple pair of pants. It's some random guy's bedroom, why would you expect him to have a crossbow in there?