It needs a network because there has to be a way to connect the devices. The other way is to use a cable connected to your laptop or maybe a tablet with some type of adapter
Once an appletv is setup (which requires internet), you don't need internet. You can airplay without internet.
I imagine the same is true for local content and a Chromecast. But, if you are doing netflix, YouTube, etc, your phone is just sending your Chromecast a link to play whatever you're watching.
I have researched the cable thing and that won't work. Most devices do not output HDMI video through the charging port (usually micro USB). I can understand it needs the wireless network but not sure why it needs internet. I've experimented by unplugging the internet from the router and it will not work. My guess is they want to know what you're watching. If someone could come up with a streaming device that didn't require internet but cast directly to the TV I think it would sell. My GoPro will stream live right to my phone in HD so I know it can be done.
