Symbols such as movie clip, graphic, etc. Can’t you make a movie without ever doing that?
Not really. First of all in order to tween properly (you never want flash to generate a Tween1 or Tween2 in your library - could cause undesirable effects) you need to have everything turned into symbols. Furthermore in Actionscript the MovieClip Class has all the goods. Then you will want to turn everything into movieclips. You could use methods like _rotation, _alpha, _x, _y, _visible and more methods in order to control your movie. You could also load Swf’s into movie clips with loadMovie action. Reduce file size by tons! I almost convert everything that I want to animate into movieclips! I load Sounds into movieclips this way my sound is in a separate swf file. Imagine you having a whole movie with 1 keyframe and nothing on stage - your file will be so much efficient.
I hope that helps.