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Answer from Ben Allison, Winamp Dev Team Lead Developer:
The “application detector” lets websites (well, really probably only winamp.com) know if Winamp is installed. We also added “winamp://” links to perform a limited number of “open such and such in winamp” kind of actions (similar to other programs that do similar things with protocols e.g. emule, aim, itunes). The browser plugin is simply a means of making sure it’s actually going to work before launching winamp://whatever in the browser. It really doesn’t do anything else – no information transferred besides the installed version. As for the benefits, umm, let me get back to you on that.
via Winamp Forums