They simply turn on some internal multiplication.

Lite mult multiplies the reflect_color with the incoming light color, regardless of angle. It has no basis in reality.

Kd mult multiplies the reflect_color with the output of the internal diffuse node, which includes the light intensity, angle, color, and the color of the surface. It has no basis in reality.

I suppose it was used a long time ago as some sort of cheap adjustment of fake reflections intensities so they could be controlled by how much light you were using in the scene.

If you assume that the general illumination hitting an object is also the general illumination hitting other objects, (not a partciularly valid assumption) and you think about the idea that real reflections would be darker if the reflected (other) objects are darker, than you can make a very loose connection that if there's less light here in the scene, there would be less light there in the scene, so you should decrease the luminance of faked reflections.

But if you're plugging a real P5+ style reflection in, these hacks are not only unnecessary, but they actually ruin the complex lighting effects we can do. Real reflection luminances will be in direct proportion to the luminance of the actual objects being reflected, based on the actual light hitting there, not here.

These values are a left-over from the P4 renderer which had no real reflections, only fake reflection maps.

One is still enabled by default because if it were otherwise, any material written back then would suddenly behave differently. Or something like that. I don't buy it, actually. Load an old material - use the value stored in it. Creating a new material, it should be off by default.

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