What is the purpose of baking powder in batter?
The baking powder undergoes a chemical reaction which produces small gas bubbles in the batter.  The bubbles break up the batter coating (sort of like the geometry inside a sponge) so you wind up with a mass of little holes each surrounded by a thin layer of bready material, and it all fries up airy, light and crispy; instead of one thick, dense, hard shell of cooked paste around your cauliflower bits.