At the moment, it is a bunch of discrete scenarios.
For example, cclive can be used to fetch a youtube video for later viewing. Should this result in a .flv file, that file can be converted to .mp4 using ffmpeg. Placing the resulting file into the directory plex serves from, and after causing plex to reindex, I can view the video on any TV via a roku. I can also stop the video at any time and pick back up from where I left off from my laptop.
That’s just one scenario. For example, my Samsung TV has a YouTube app. I can bring up YouTube on my Galaxy S3 and “pair” it with the TV, and watch the video on the TV.
For many videos (not including YouTube, for reasons I don’t quite understand), I can stream the video directly from my phone to my TV using DLNA.
Going back to the mp4 that I fed to plex, I can directly copy that file to my cell phone (for example, using USB) or iPad and watch it later.
Plex is a great add-on for a Roku box. If you do try out Roku, I recommend that you load up Plex on a server-like system. The combination is great because Roku is very hands-off (consumer-friendly), and Plex adds the ability to watch anything you want.
I’m not sure if your video toolchain has evolved since this post, but I tried using cclive for a while and found it brittle (at least on OS X through MacPorts.) youtube-dl seems to do everything cclive does, and more, and has a more active community around it. [link]