The purpose of the Late Roman Draco standard.

[ValentinianVictrix]

I'm not implying that the Eagles represented the Emperor, I've read in a number of books that the Eagles represented the might of the Legiones, were in fact the physical representation of the Legione. It would have been a good way of indicating on the battlefield which units were legiones and which were auxilia, something that during the Late Empire period may have been something of some importance as some historians claim the arms and equipment of the Legiones and Auxilia were the the same at that time.

What symbols were used to denote the presence of the Emperor or to depict him if he were not present.