This style has mad significance. First, if you don’t set it on most containers, they won’t have a background. If they don’t have a background (fill/rect/what have you), you cannot use them as drop targets since there is nothing to detect a drop on.
Secondly, as you’ll see in my next post, if they don’t have a backgroundColor defined, they don’t draw one, so at runtime (at least as I’ve found), you can’t use setStyle(“backgroundColor”, 0xFF0000); there’s nothing there to color. I think by setting it at authortime, it draws a background, and from then on at runtime you can then use setStyle to color it.