
Originally Posted by
Centaurion
From my own experience, this is to invite attacks.
The AI checks if you have enough forces to defend your settlements / forts. If you only have 1 cheap unit in the fort the AI decides to attack since it thinks that you don't have enough units to defend your settlements (even though you have several stacks nearby).
The best way is to have a whole legion sitting in a fort at a choke point. During my campaigns I have never been attacked by the AI when I have a legion sitting in a fort. What happens is that the AI moves a stack close to my border, sees that I have enough troops and pulls back. If I only had 1 cheap unit in the fort, it would have attacked.
EDIT: In a roleplaying way, you can say that the legion is protecting your borders and at the same time pacifying the newly conquered lands. The Romans had several legions garrisoning newly conquered regions, and protecting the borders.