Monkeys fling their poo.
Humans are much more advanced though. We turn our poo into an idea, and turn the idea into a word, and fling that. There are several advantages to this approach. First of all, your hands don't get dirty. Second, you never run out of ammunition. Third, you're not limited to what you've got on hand--you can get all kinds of creative with this concept.
I think this adequately explains both why we use foul language, and why such language is so jarring and powerful.
So... bear that in mind when cussing or being cussed!