The leaf was beautifully animated. Very well done.
As for the whole coding thing. You would only use coding in animations for just a few reasons:
1. The virtual camera. This is already made for you though, so you don't really have to worry about it.
2. Effects. There are some effects you can do with actionscript. Adam Phillips uses awesome new ways to use actionscript in his movies all the time. Such as those bugs flying in Waterlollies. They were made with actionscript.
3. A menu and scene selects. If you want to have a scene select in your flash, then you'll be needing a menu, so they'll have the option of either playing the movie, or viewing the scene select.
4. Time controls. Pause, play, fast forward, rewind, next scene, last scene. All of those things would be done with actionscript as well.
Basically, for most animations, it's just used to add more features to the flash, such as menu's and stuff.