Why was the Persian standing army an important factor in Persia's rapid growth?

The Persian Empire stretched from Asia Minor to India, and included many conquered nations. A strong army was important to maintaining the Persian Empire's power for many reasons. Firstly, the army helped to conquer to new lands for the empire. Secondly, they helped deter any possible uprisings or revolutions among the conquered nations. Thirdly, they helped maintain the peace throughout the empire.