This is a really difficult question to answer to your satisfaction, because the answer is all and at the same time, none. A shield is far more than a wooden counterweight held at your side; it's meant to be used actively, to push, to bind, to bash and trap. Depending on the size, it's used differently of course - a buckler is better suited to fencing than a Roman scutum, and the latter is much better at fending off arrows and forming a solid formation. At the same time, I'm not aware of anyone using it solely; why would you leave your other hand empty when it could grasp a weapon?