WebRTC doesn't add anything for his usecase of playing local files. WebRTC is an API to let the browser stream data (including video/audio) over the network. If the browser can play the given video by supporting that codec then you can play it using a HTML5 <video> element directly from local disk. No network communication needed.
Ah yes, didn't think about the use case of unsupported codecs. Though at that point I would argue it would be easier to convert the file with ffmpeg, etc, then to use a custom decoder in the browser.