If you have Flash installed, this add-on is not useful to you.
This will not change YouTube, Vimeo or Dailymotion. You won't see any change there.
This is to replace the legacy Flash embedding of YouTube, Vimeo and Dailymotion videos on third-party website. Some CMS still do that, older content didn't get updated, etc. Many reasons but this break the open web.
So "no-flash" will do what it can to fix that. It is not a silver bullet, but it works in many case - YouTube being very common.
Also, if YouTube insist that you need Flash to view a video, there is unfortunately nothing much that can be done with this add-on.