The SDK Documenation says prop_physics_multiplayer is required for online maps because prop_physics are calculated clientside. However, Lockdown and Overwatch don't use them. What gives?
Use the prop_physics_multiplayer when you want physics objects but wont have them much for player interaction. So if you want some fancy effect, say falling rocks etc. but it wont be an intergral part of the map when it comes to what the player does/where he goes. Just eases things up for the engine then.