When I was a Christian, I prayed a few times and thought that those prayers had been answered but I later realized that I had conditioned myself to expect that result, and that was precisely what started my path to atheism. So yeah I'd say human history's greatest placebo, and I don't mean "greatest" in a good way.
Sure. Nothing wrong with the "god" idea if it's like the Native Americans did it, but most major religions were designed for population control/domination.