Many airlines used it for that. When the 747 first came out, pricing was still under control of the Feds (this is pre-deregulation). Initially, if you offered a 747 from say, Chicago to LA, and your competitors didn't, you probably gained business. But when everybody had them and was doing that, you had to do something to differentiate yourself for the exact same airfare. So some turned them into bars, lounges, and yes, American offered piano bars.