Make that having a good diplomacy in a haven allow you to get new interactions like:
-Discount in trading;
-More volunteers to recruit;
-Ocasionally get some resources gifts;
-Get special missions to improve relations with the Major Faction;
-If in high diplomacy, they can have a chance to deploy in mission near their haven to assist you.
That would need that Haven Leaders make requests/missions to you, to you be able to increase your reputation(its in another ticket)