I know, aren't they awesome?
They were made to look like that for a few reasons. First off, they wanted to appear intimidating both in battle and to their own troops, as it helped them to gain the respect, or fear, of their men. Secondly, it made them appear authoritative. And third, the large (red? or other colors depending on the depiction you see) crest made it easy to find your Centurion on the battlefield.