Were Psychoactive Plants Designed for a Purpose or Just random evolution at work?

I do think they are designed as a cure for illness... just like marijuana for example. they have been used centuries before to cure any diseases. but nowadays, it seemed that the herb is being used in the wrong way... its so sad.and just a random note. My sister has been buying Salvia Divinorum behind my back and she purchases it at this Salvia Divinorum site {removed link, which was very spamish - Adminnemooseus} and she says it does wonders for her. I'm not really sure if how I should stop her from using this one... although I've heard its good but still, it's a drug. hope somebody can suggest any alternative treatments thank you.