I'm not familiar with WebRTC. What's the use case there? I can't remember ever wanting to create an in-browser p2p connection on my local network. What would it be used for?
You'd want to use it any time you want a high-speed network connection with another user. For example, a multiplayer game or video teleconference.
Others have pointed out this behavior has changed in Chrome 48. You don't get the local IP unless the page asks for access to the mic/camera which the user has to give permission for.