The reason (hopefully) is to give the user some simple variations.

It would be like me giving you a shader option with an initial value of 0, which does nothing. You could turn it to 1 and it does something.

For example, I gave out a light set that had a rim light already set up behind the figure (tricky thing for some people apparently), as well as two main lights - one for a dramatic facial closeup and one for a full body portrait. All you have to do is turn one off and turn the other on to switch.

Why not two light sets? Well suppose you wanted to slightly adjust that rim light while working on a full body shot. Then later you want the corresponding facial closeup. If they were in separate light sets, any modification you made to the rim would be lost when you load the closeup light set.

So by combining the two scenarios, you take advantage of the common factors which you can then adjust ONE time instead of two.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)