Why do games need bosses? We don't know. Neither does Valve's Chet Faliszek.
In Left 4 Dead, he told Gama, "we don't make evil scientists that have created a zombie infection to stomp out the USA - the evil Russian scientists or whatever post-Cold War enemy you want to use."
Zombies are zombies, said Chet.
"I think everyone knows what the zombie apocalypse is," he said. "They register what it is and they, playing it, make the story, because, since you're so close to each other when you play, you see the same things. You have the same experience. You can talk about it afterwards, and it's not like, 'You missed this. When this crazy thing happened, we were over here.' Instead it's, 'We were all right there, and we all saw that.' That's much better writing than I could ever do."
More through the link.