It doesn't do anything functional day-to-day. It's just there to mark the positive terminal on earlier WK2s and to give you something to grab onto. Problem is, after you grab onto it a couple times, and especially with a few years on the plastic... it's done. Stupid design. Cheap materials.
Boyfriend did a fine job as long as he got an AGM battery and reconnected the vent.
It clips onto the positive terminal to align it with the battery . While you dont really need it, without it installed its possible to turn the terminal in such a way that it could make contact with the body and cause a short.