Switching light on and off, has many benefits and a few things to keep in mind. When used you basically double the size of your light data. The game must load information for the lightmaps with the light ON, and lightmaps with the lights OFF. For some players, this will mean, it takes longer for the level to load, and require the engine to keep track of more information.
Best example for Light Groups. SAMPLE_DM , A Lesser Fate has that switch in the lower sub basement that turns off all the lights. That is done with this light group set up. Go study it.
If you've made it this far into tutorials, you are already proved yourself to be diligent and hardworking. None of this stuff is complicated. It just requires a bit of tenacity.