I often use the built in drawing tools to display AOE effects, but for some it would be better to be able to have a filled in area rather than an outline. For example, here's how I would currently display a spell that creates a cloud of fog around a character: The solid lines mark the edges of the AOE well enough, but look at that poor Otyugh! Sliced in twain by a solid line, but If he were to be placed above the line he would obscure the borders of the AOE, which while fine in this case could be an issue for larger tokens. Now behold! With the power of opacity the area of the fog can be filled in, making it look better and sparing larger creatures from being bisected by solid lines. For the second picture I filled the shape with the same color, removed the border, and moved it to the GM layer so I could use it's opacity for the screenshot, but obviously players can't see this. I'm sure there's many other uses for drawing and object opacity, this is just the example that came to mind.
