There are times when you really need to have some object visible during edit as a marker that you don't want visible to visitors.
Right now, you can use Audio Emitter or Trigger Volume objects this way, and people are starting to do so for some scripting purposes. I'd like LL to formalize this with a boolean setting.
If the object has a volume, that volume would be there at runtime, but its visible model would not. If the object has no volume, it will have no representation at runtime. If the object is dynamic, it would still be invisible but otherwise behave as any other physical object.
One use case would be invisible walls and obstacles with surfaces that make it easy for the designer to precisely place and visualize. Another use case is a teleport target for a scripted teleporter (no physics). Another is survey markers showing the bounds of some range used in a scripted game (no physics).