The Roman soldier carried standards into battle so

so they knew were they were on the battlefield  and to communicate in the battlefield.

Explanation:

The Standard, also called as Signum or Signa Romanum, used by the Roman soldiers was a flag or military ensign which identified a particular military unit.These standards were usually attached to a pole or staff and the soldiers carried it to identity between a cavalry and infantry.The Roman soldiers carried standards into battlefields so that they knew they were on the battlefield  and to help them communicate in the battlefield with each other.