This is mostly a performance thing. Comparing static/stationary/moveable,
as in the mobility documentation you linked it says
-
Static:
This mobility is reserved for Actors that are not intended to move or update in any way during gameplay. -
Stationary:
This mobility is reserved for Actors that can change during gameplay but not move. -
Movable:
This mobility is reserved for Actors that need to be added, removed, or moved during gameplay.
Static is not meant to update or “change” (as in swapped for another thing) at all. In the same documentation you will find their mobility allows different levels of optimization related to how the lighting system works. If you’d place 100 buildings on a level you’d place them as static mesh because they never change or move.
What I assume a case, is that you should use Stationary actors on for example a city building game where you can place and remove houses of different models on a map, which (obviously) never move around like a character would.
When a house is built or removed , the lighting system should then update to match this state