Power
This depends on the clients you work for. If you're part of a marketing communications department, you have some power, but not much, because you're there to handle the same types of work over and over, year after year. However, if you work for top-notch clients with important things at stake, you can make or break them. It's a lot of power to wield.
If the client treats you right, you work hard to keep them going on the right path. If the client disses you, you have the skills to concoct something that will make their empire collapse (not that you would ever really do that, now would you?).
Tread lightly on the revenge stuff though. It's like when you were a kid and got to go to a friend's house to play. If he made you so mad that you broke something of his to get even and then you got caught, he wouldn't ask you over to play again. And that would suck, because his mom made the best tuna salad sandwiches.