WebRTC is very, very hard to code for. But if FFmpeg abstracts that complexity away, then WebRTC becomes much easier to add to a software project wishing to benfit from that which WebRTC offers.

I guess I still don't understand. You don't really "code" with ffmpeg. It just is used to transform media formats or publish to a public streaming endpoint.

All of ffmpeg’s functionality is accessible from C (and transitively most other programming languages) via libavformat, libavcodec etc. FFmpeg supporting WebRTC means that projects using these libraries gain support for WebRTC in code.