If its cleats near the clew, I think you are refering to the leach line. If it is the leach line, then it controls the flutter of the leach, typical on a big jib. For our jib, which is in the 90 s.f. range in light to medium air we don't need to tension it. In medium to heavy air we do need to tension it. If you over tension it it will curl the leach inboard, hooking it. If there is another use for it, hopefully, someone else can fill us in.