This was very helpful Tim!
After I played a bit around the Shooter Game Main Map, I see that the Level Panel works like Layers or Folders in Photoshop, a way to put different assets for your level to different Maps, for organization and as you mention, performance as well.
I think that also solves an issue we had, that only one person could work and commit to a level. With this, one artist can light the level while an audio guy will place the appropriate sounds.
Another great tool from Unreal.
Thanks a lot for your quick and detailed answer!