From chewing betel leaves, to magic mushrooms, and of course the fermentation of alcohol, forebears used these for their medicinal, transcendent and stimulant properties.
I once had an archaeology trip to a forsaken lump of rock in the bum end of Wales, which was a prime ceremonial stone axe quarry site. The lecturer suggested that the continued abundance of magic mushrooms in the area may have been present in prehistoric times as well, making the site renowned for visions, and therefore the nearby rock was deemed special.*
So what, you may think? Well, the next time someone wonders "Why did they build Stonehenge etc?" the answer could amusingly be "Cause it was a great spot to pick up drugs" or simply "The rainbow unicorn told them to."