My answer to this question may have been deemed too simplistic, but that was the entire point of posting it.
The other answers we're nice and informational, but in my opinion they breezed over the very simple question OP actually asked.
OP basically asked (paraphrasing), "what is the connector on the left of the plug for?"
P.S. My experience on other SE sites is that deleting answers that do at least attempt to answer the question is not generally correct. Honestly I would rather have my answers left to get downvoted than have a mod delete them aggressively.