The Flash plugin helps with handling Flash animations and movies. It adds a capability to the editor to add Flash files and ways to properly place them in your page.
Once a Flash animation was added to a page, that page requires the Flash extension. However, some users may not have Flash installed and in such circumstances you probably want to let them know why your page is not fully functional.
Also in some cases, website administrators prefer to show an exciting image that shows that there is a movie that one can watch, but without actually loading Flash by default. This plugin allows you to do so as well.