When you're creating online applications you often want to mimic the tools people are used to from the desktop. The classic marching ants selection marker is a good example and even easier than before, now that the lineBitmapStyle has been added to AS3.
this.graphics.lineBitmapStyle(ants, matrix, true, false);
this.graphics.drawRect(downX, downY, extentX, extentY);
Fullpage Example Click and drag to make a marker.