How is this different from something like BubbleuPnP or Plex? No GUI? I'm not a developer so a lot of what I see when I click through to Github is gobbledygook.
The benefit of this library is being able to play media on multiple chromecasts on the same network. So using this, a developer could create a Plex-style app for casting to multiple TVs. One use case could be digital signage at an event, where you'd want to control all TVs from one console.
