Charlie, the purpose of the box is as stated by Marcus. Here is a picture of a couple of weep holes in the box (circled in red) that lets the water drain. It is possible that a previous owner added the weep holes on my boat if they aren't on yours. I believe that there is a thread somewhere on this site that discusses problems with water collecting in the box and seeping down below. On my boat, a previous owner fixed the problem by laying some epoxy inside the box without covering the weep holes.
We use it for Winch handle storage. When our boat was designed it came with the main and jib winches on the mast so it is perfect to store the handle there
I have the same configuration and same weep holes. Prior to fixing the leaking issue, all ours collected was water andhttps://www.google.com/search?safe=...a=X&ei=M83bU5WuKNDfoASruYLgDA&ved=0CBwQvwUoAA occasionally a sail tie. The leak below found its way into the head liner above the head. Once fixed with epoxy and some gel coat, all is dry.