CommentStreams:7f791c570b31328aaded3e187cf9f157

Initially it only serves as a signal from the guild master to the guild members which square needs the most focus. If you buy Battle Banner from the store it increases the total power of troops in that square by 10% * (number of stars). It is unclear if Battle Banner increases the number of troups added before setting the flag or only ones added after.

Admittedly, if you have an external communication channel within a guild you don't need that flag. You could also use a different approach - e.g. agree that guild master places some troups initially into all of the focus squares.