Originally Posted by
Sheridan
It's for repopulating desolated regions (those which are razed by the player/AI). Which of course isn't a great or accurate representation of the colonizing that really took place, but it's one way of doing it. Recolonizing costs both gold and manpower, the latter from the army which you do it with, and it can be very expensive in both regards, so the army tradition bonuses here a pretty powerful in that sense.
Edit: Hellenistic traditions now fit with new icons, and the Roman tree has the final two done now (thanks to UMCenturion and Saffron)