Well, the whole point of a diplomatic interplanetary body is to have a common forum of communication to soften their relationships over time, by allowing dialogue and cooperation to occur at some level in a fashion doesn’t involve shooting phasers and photon torpedoes at each other. Even if the shooting still happens from time to time.
It’s a parallelism with the UN. All sorts of countries who hate each other are part of it and sit at the same table, at some unequal level even. They still shoot each other and commit atrocities towards others and their own people from time to time. But still, the point is that the table exists and is available. It creates opportunities for dialogue and cooperation, that wouldn’t exist otherwise, that create opportunities to solve conflict peacefully and soften their differences over time.