The SHOUTcast Flash Player was a lightweight .swf file embedded into a webpage. It acted as a bridge. You didn't need installed software; you just needed the Flash plugin (which, at the time, had 99% browser penetration).
If you were building a website between 2005 and 2015, there was a 90% chance you needed to answer one specific client question: "How do I get that little music box on my sidebar so people can listen to my radio station?" shoutcast flash player