Neutral planets could create buffer zones between empires that would have to be annexed prior to an invasion and also act as way defending yourself from an enemy.
Neutral planets might have key exports and imports making them significantly more valuable as trading partners then they would be to conquer.
Neutral planets might encompassing many unique bonsues things for instance a research colony might provide a free technology once you've supplied them with a set amount of trade goods.
Neutral planets could be used to fight proxy wars. Why attack your enemy when you can get a neutral race to do it.
Rare planets could be sold to private individuals and corporations for massive one off bonuses becoming neutral worlds.