Coasters or "beermats" can help protect your table or table cloth from stains or burns from hot beverages. If placed on top of a drink it can protect it from contamination. But, did you also know that in many typical German breweries the waiter will automatically keep replacing your empty beer glass with a new drink UNLESS you leave the coaster on top of your empty glass to signal "done - enough"? Prost (Cheers) to that kinda service!