Why does 0 opacity use GPU fill rate when setting invisible does not

I ran some tests to figure out what affects the gpu fill rate. I noticed that 0% opacity sprites use GPU fill rate, but setting them invisible does not.

Is there any reason for this?

Is it because 0% opacity can be used as masks with some webgl effect? (but that would turn the layer to render it's own texture, so it could be solved)

Think of opacity or alpha as a color. Changing the alpha value is like changing the red or the blue or the green value. The alpha value being 0 or 80 makes no difference to your GPU. The visiblity toggle on the other hand fully disables rendering of an object.

I understand, this makes sense, thank you.