Being poisonous, itself, is not super beneficial to the specific animal. It's good for the species cuz anything that's about to start eating your kind will die, but it still requires that first sacrificial lamb.

So a second level of defense is advertising. Hey, look at me. Yeah, there's no way to confuse me for another animal. I'm definitely the poisonous one. Don't even try it, I would rather us both live than us both die. Go eat Frank over there.