On what it should do -
You use IR or a final render to adjust LightMix to something you like. However, the number of layers in LightMix is causing high memory usage, or denoising is taking too long, or you made some very large adjustment to light intensities and now noise shows up in the LightMix. So you use >Scene which sets the scene lights to match color and intensity as it would be after the LightSelects.
Now that the lights in the scene have had their colors and intensities altered, you re-render - you can delete the LightMix and LightSelect layers to save memory / denoising time, or reset them to 1 and white if you were just wanting to have a new starting point where you didn't have to make such large adjustments that it was introducing noise.