It has been a while since anyone has made an attempt at guessing, so I suppose a clue is in order. Think of it as an anchor, just not the primary definition of an anchor, something to hold a boat in place. However it may be used on a boat by some of its users, but would serve no purpose whatsoever to some boaters.
It's not for wrapping rope, cord, line or thread. It can be used with at least one of these and other things similar, along with other items not even mentioned yet.
Earlier you said it had nothing to do with a rope, so it must have something to do with a line or thread. You said it could be on a boat or not. Must have something to do with fishing lines or reels.