Because Liu Village is certain to need new inhabitants in the future. The population of a few thousand, although it may seem plentiful, is far from sufficient.
Expansion is an inevitable trend and the key to the lasting prosperity of Liu Village in the future.
When that time comes, opening and closing the village will be too troublesome, and it won't be feasible to keep an eye on it all the time.
In Jiang Hui's mind, he planned to add a precondition to this formation, giving it a certain degree of autonomy for decision-making.
For instance, if someone with a sincere heart truly comes with genuine intentions wanting to pledge allegiance to Liu Village, they should be able to pass through Paradise safely and enter Liu Village.
On the contrary, if someone harbors other intentions, they would be sealed within the formation, lost in a dense fog that can isolate all forms of life.
Then, how to deal with these people would be decided by Jiang Hui.