Other people have already mentioned how the government runs the alcohol stores and (IMHO) they do a pretty good job of it. I live in Texas now and when I go back to Ontario to visit my family, it does seem weird that I can't buy beer in a 7-11 or a grocery store or in a drive through shop (not sure these are a good idea).
Being the sole seller also means that they can negotiate ruthlessly and drive prices down to maximize profit for the people of Ontario. I don't know if they will do that, but it sure helps.