Everybody knows that tobacco leaves contain nicotine, but why?

Wild tobacco, is essentially a weed. As such, it is accustomed to growing in soils and environments that are not the best for other plants for varied reasons. Insects are one of those reasons.

Tobacco plants actually produce nicotine to kill bugs. Thats right. You are addicted to insecticide.

Insects that consume tobacco leaves are poisoned in several different ways. First, they are exposed to high levels of heavy metals. Tobacco plants leach all they can from poor soils and as such pick up all kinds of minerals. Heavy metals are known to damage nerves. Secondly, they consume a potent stimulant, nicotine, that acts to suffocate them by over stimulating their nervous systems. It acts like strychnine, a bitter poisonous alkaloid (C21H22N2O2) does in rat poison. If a bug can't breathe, it can't eat you. Ingenious.

Eating tobacco can have a similar effect on humans. Hundreds of years of selective breeding have made the nicotine levels in tobacco skyrocket. If you eat the entire contents of a pack of cigarettes (don't), you can potentially die in the same manner as a poor bug. Suffocation.