The Meaning of Life, the Universe, and Everything.
This is a potion that'll act as a sort of poison to fire based mobs (blazes, for example). You'll be able to make this potion with lapis lazuli, and there'll be splash & lingering variants as well. Obviously with the amount of mobs based around fire as of now, this won't really have many uses. So I don't expect it to get much support until maybe later on.
Yes- the only difference is splash potions of water will od more instant damage and this would do poison? They're both specifically for fire based mobs and I don't think 2 potions just for fire based mobs would be necessary.