Why were ancient Roman soldier shields so important?

Over the centuries of ancient Roman wars, the shield has had many shapes and sizes.

The shield was a vital piece of military equipment for a Roman soldier. In battle the shield protected a soldier from spears and javelins thrown at him. Arrows and slashes of an enemy's sword also provided protection with a shield.

When a city or fort was under Roman siege, the Roman soldier's shield also protected him from objects thrown down at him from fortress walls.