I would wager it's not similar to the cord on samurai helmets, as those had a separate faceplate and head/neck protection. The cord on the samurai armor is what holds it on, but a great helm / sugarloaf / similar styled helm can not be knocked back off your head from a well-placed frontal blow like an untied samurai helmet could. My bet would be on Sir Brian's comment, that it is likely for tying it off and being able to carry it without having to hold it in their hands. Sir Edward made reference to helmet straps worn "around the neck" on some of the earlier period literature last week at MDRF, but I can't remember the exact source.