When I was first learning how to build websites, Flash was something you needed to learn. I purchased a copy and through the entire interface more complicated than it needed to be and in general didn't care much for the results. I quickly moved in another direction.

Most of the videos you've watched over the years have been Flash or worked through a Flash player. That's changing though and the writing is on the wall for Flash. It can still do things you can't yet do with html5/css3/javascript, but it won't be too long before that's no longer true.