NPCs are the lifeblood of the campaign. Or at least they're that volume of the campaign's lifeblood that isn't being pumped by the players. Okay, okay. I admit it: I'm struggling to make this metaphor work and it just isn't happening. I'll start again.
NPCs are the poor suckers with whom the player characters have to deal on a regular basis. Heroes, villains, authority figures and complete nobodies; all of them have their part to play.