When I am in front of a Wall Conveyor and it has the Plating color, then I can look through the hole, attach a belt to the rear of the belt connector and connect it to something in the distance.
This way I can look into a room that has a splitter and connect it without going into the room.
This does not work when the Wall Conveyor has the Sheet Metal color. Then it's almost as if the game doesn't see the hole as a hole. I can only connect to the front/outside of the conveyor wall, but not what's inside/behind it.
This is annoying because I want to use different wall-colors in complex factories and Sheet Metal color now forces me to remove walls to make connections (or do belt upgrades) in closed rooms.