I'm confused. It cannot be a coincidence that substances exist on this earth that alter our consciousness? Why not? There are all kinds of substances found naturally that can change the way you feel, change the way your body acts, kill you, make you hallucinate, whatever.

Is it proof that there has to be a purpose behind them?

I forgot what the term is, but is this not like the fallacy of believing there must be a creator, because there is this wonderful planet for us to experience? But if we were not here to experience it, we would not know it was missing.