What is the point of THC?

Why does the cannabis plant contain tetrahydrocannabinol, or THC for short. I know the ingredients in marijuana and how it gets you high and all but WHY does it do this. The high comes from the THC but speaking from an evolutionary standpoint, why would a plant evolve to the point where marijuana has and why would a plant need to evolve to contain THC? What is the point of that in marijuana and how come marijuana is like the only plant which gets you this high? How come other plants don't? Thanks=)