Here's a real answer for you:
While the Roman empire was very big and great and famous, no empire can sustain itself without backup. The Roman army is the "backup" for the empire. Without the army, any other country could take over the empire or destroy it, and the Romans would be unable to resist or put up a fight. And that's why all powerful nations and empires need armies. Of course, the Roman empire was TOO big, and the army was too small to secure the Roman empire's massive borders.
Hope this helps!
