Encoding videos on the desktop with the native FFmpeg code will not only yield better performance, but I can't think of a scenario why you'd want to use as a web-based service.
Create screenshots? Really? What a complete triviality. I guess using standard software and hitting the PrtSc key is too difficult in comparison to using the browser for everything. Making screencasts is a different issue, but also one you'd do locally.
Sorry if I'm not so passionate.