Oh! They're to make your baths nicer! You know, they're just fun and they help make the bath a little more interesting than just sitting in a bunch of water, and they can usually help soften your skin a little. And sometimes they'll have scents or glitter so you can sparkle or smell nice after you're done your bath!
I've never used one, but I bet it would leave a glitter ring, too. Hopefully a cleaning cantrip would be enough to clean that up at least.