The off hand is used for balance during fleche or a lunge when it is extended backward - normally kept curved upward behind the head away from stray hits with the epee or foil (which smarts like mad if you get caught across the knuckles.)Actually no. It is held at the ready somewhat tucked to the side. The only time it readily moves back is in the lunge.
Part of holding the arm back also helps with presenting a smaller profile reducing the target area you expose to your opponent.
Rapier and dagger/main-gauche styles tend to have you leaning forward, your presenting a far greater target area to your opponent.
As I mentioned in my previous post, shield use is great for battles and ventures in to the wilderness, but carry it around an urban environment is going to land you in a heap of trouble with local guards/militia, the same goes for pole arms and unsheathed blades.