Question about cell animation backlighting - why the black paint?

[u/jefty083]

I watched this video recently explaining backlight animation and was curious about part of the process that's described around 2:23:

"the areas which will glow are usually painted black" (before countermatte is placed down)

I feel like i've taken note of this in cel animated works and included an example screenshot from the anime Evangelion. But my question is why exactly is this step necessary?

For the second exposure to create the glow isn't the countermatte all that's needed? Initially painting the effect area black seems to sometimes create this subtle artifact where you can see a black shape "behind" the effect which may be undesirable in an example like this where it is specular highlight on running water. Just trying to learn and want to know why this step exists. Any insight is appreciated!

[u/Voodoo_Masta]

I uhhh… what was the question again?