I mean, I know the Indians were smoking in the Americas for hundreds or thousands of years before Columbus, but who decided it would be a good idea to shove some dead leaves in your face and set fire to it?
I’ve got to tell you, most of what I’ve read about smoking and other plant use as a social tradition has been pretty general and not too scholarly, but…
Plants all over the world have mind-altering ingredients, many of which fall into the alkaloid family. In addition, plants all over the world have medicinal properties. Tobacco in its pure, unprocessed state, is claimed to have both, by some herbalists/proponents.
There are all kinds of ways to prepare plants and herbs for our consumption, especially medicinally: steeping bark or roots for tea, grinding stuff up for poultices, and on, and on, but of interest here–burning or steaming leaves, flowers, etc., for direct inhalation. Anyone who’s been able to suck on ice chips when they couldn’t even keep water down will tell you that the mucous membranes in your mouth do a pretty good job of substance delivery (the ones in your nose do too, think Vicks).
I have no idea who invented the pipe, or when, but maybe you didn’t know that tobacco was also used by Native Americans in both sacred and healing rituals as part of sweat lodges–the plants were thrown directly onto a central fire, which is a little less concentrated than holding a bunch of burning leaves in your face.
This only hints as to the “why” part of your question, since we probably won’t ever know the complexities of any single person’s approach to drugs. Think about all the different reasons why you and/or your friends might drink coffee. But cultures have been using plants to feel better or feel different since about as long as we can determine, some medically, some religiously, and some just plain old recreationally. The modern day cigarette is pretty far removed from the use of unprocessed tobacco (which I personally don’t think is any healthier). Remember that a substantial amount of litigation against tobacco companies has been addressing their fiddling with nicotine levels and additives which ensure a stronger, more addictive product.
my wag is that they burned the stuff to clear fields or to keep warm. They noticed some mind altering effect by being around it and liked it or grew to like it. They made it in a compact form. That’s my wag.
This link seems to have some good information… at least for tobacco.