What's the point of debuff potions?

Assuming you're talking about the Enhanced Edition of the game, those debuff potions are a forgotten remnant of when you were able to combine them with grenades and arrows to allow poisoning from afar, back in the classic version of the game. However, the ability to poison arrows and grenades seems to have been removed in the EE version of the game.

Because of this, these potions don't appear to have any real effect anymore.

According to this wiki page you can craft arrows that debuff your enemy in combat with these potions.

For example combining the Debuff Perception Potion with an Arrowhead results in Blinding Arrows.