In SOW the flag bearer is very important.
Why is this? The flag represents the centre of the formation. The engine concentrates on the flag and ignores the end points of the formation. The flag is the point which ends up on the destination arrow. When you pivot etc, the flag is the point of reference.
The flag is used for Line Of Sight calculations. For example. You spot part of a brigade on a ridge but the flag is not showing as it is on the blind side of the ridge. Your ai cannot target the brigade as it has not seen the flag.