I was reading & searching information about shields during Roman republic and one thing come to my mind.
In melee fight you can be stuck in one place with enemies in front and comrade in back.
You want to have your shield in the better position possible.

Perhaps the cord inside of the shield was used by the right hand to keep the shield in place ?

if your talking about the roman shields the cord is to sling the shield on your back with shields like the aspis its purpose is the main grip so that when you place your arm through the strap (the greek name escapes me) and pull the cord tight it locks your arm in the proper position to both move the shield and brace it on your shoulder. the Roman shields to my knowledge was mostly center grip shields so using the strap to grip it would be pointless

The handgrip (antilabe) on the Greek aspis is separate from the carrying cord. As is often shown in artwork, on my shield the grip is thick and braided, and runs between the two bronze loops. The carrying cord starts and ends at those loops, but is *not* part of the grip itself. It is always shown loose in artwork, so there is never any tightening of it when the shield is carried. With the forearm through the central band (porpax), the inner rim of the shield rests neatly on the shoulder, supporting the weight. The cord is only used on the march, to sling the shield on the back. There are clear vase paintings that show this.

On the other hand, I am currently working on Macedonian shield, which is basically the bowl of an aspis without the flat rim. In this case I will be using the carrying cord to support the shield in combat, since both hands are needed to hold the pike (sarissa). So far I have installed the same sorts of fittings that you see in my aspis, but it may take a little fiddling to get the cord just the right length.

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I'm painting it tonight, though, so you'll have to wait for those experiments!

The Roman scutum also had a carrying strap, at least fittings have been found to imply that. But again, that was only for slinging it on the back on the march. In battle, it was held by the single central horizontal grip.

Matthew

sorry i was talking about early roman shield like greek hoplite one.
ho, ok it's just to carry it on the back :p, my bad ^^