The purpose of the Late Roman Draco standard.

[John Conyard]

Certainly standards could be used to give visual signals such as to advance and to retreat. But also to set the speed of maneuver, especially important for cavalry, both in Arrians games but also in the field. Hearing orders can be difficult in any armoured formation, but especially in a "cavalry" helmet bouncing around on a horse when you are well back in the file. But by watching the draco speeding up, slowing down, wheeling etc. you get an indication of what to do imediately in terms of speed as well as direction. The tail of the animal makes it easy to see and helps gives a visual sense of the direction to follow.