I'd say the major advantage was you had an IDE that had a canvas to draw on, a visual timeline to aide your animation, tweening, powerful scripting - much like

I'd say the major advantage was you had an IDE that had a canvas to draw on, a visual timeline to aide your animation, tweening, powerful scripting - much like Swift, and compilation (ship a self contained SWF binary). For me the canvas was powerful, I could use a wacom to draw, tween motion between frames and visualize the whole thing quickly with playhead scrubbing, and keyframe tweaking etc. I don't miss Flash but it would be really interesting to see how it would be used in today's mature webscape.