Temporarily eases relations with the target nation.
It works much like the Bard Archetype ability, Soothing Song. The Diplo Dervish will temporarily raise your relationship with another nation to a Blue Face. It very useful when playing economically, so you can keep your trade routes up. It also helps when trying to finish with a Diplomatic takeover, simply making Alliances when playing religiously. Even military nations may find a use for it; they could keep an enemy nation at peace for a time while they finish off another nation. However, most players who use military action in the Civilization Stage typically do not socialize in the Creature Stage unless they are going for Knight or Zealot archetypes as either an Omnivore or Herbivore respectively.
It can even be used offensively as an economic nation, to trade with and buy out cities from a superpower that is irreversibly at war with you, as well as distracting them with an allied superpower.