well, here is my take. it served a purpose many many years ago, its a strong industrial fibre and they knew of its use. so they took that. they also noticed a strange thing, its excreted a chemical that people finally found and used for their own purposes, and liking it so much propagated the plant as a crop. throughout the years of evolution we have changed our perspective of what the plant can do for us, it has evolved with us, as have other plants, corn in particular, look at the history of corn and you will understand the recent history of MJ. as for the original reason behind the thc production. i seriously doubt that it was a major factor, the original hemp plants were too low in content to be of use like that, they just produced it, and people took advantage of it to make the potent strains we know thousands of years ago. but it probably had something to do with a mutation, and its just a mutation that people in particular wanted to exploit so we did, but many plants create many chemicals that have no use whatsoever and we ignore them. im sure oak trees create chemicals that we could extract, but what would they be good for??? we just found one and played with it long enough to create effects that were more desireable for us. im sure we could do the same with oak trees and find ones that make more acorns, which are edible seeds, we just typically dont eat them is all. corn used to look more like a cross between barley and the seed pods of grass, and we early in our history did our genetic breeding to create a plant with a thick stem that grows easily and creates a highly nutritious full ear of corn. so it was just by luck that we found the weed plant, its by our desires that it does what we on this site get our effect from. if we had never interfered who knows, the gene for producing the thc might have eventually disappeared completely. there are plants that produce poison in their seed pods, what would the point of that be if the point was to spread the seed, its all intertwined with human nature, and evolution. we have been affecting evolution since we found out how to change crops to do what we want them to do by finding the correct traits we are looking for. essentially, we have been playing god for millennia.
when you learn to walk on the planes of existence, you can learn to run on the planet of earth