This flag would stop damage from being applied the sitter on that prim.
A use-case I run into is in combat vehicles that need to track their own damage and, because of the unknown order of on_damage events, rely on final_damage to see how much damage ended up getting through.
Any leftover damage in final_damage
must not
be applied to the sitter, otherwise they will most likely just die instead of the vehicle absorbing said damage into their own hitpoints.