Since the cards figure to break even in the long run, without the possibility that someone is bluffing each player would have the same expectation, and when all was said and done, no one would win any money. But there are winning players and those who lose most of the time. Bluffing, after all, is merely a form of deception. And deception is an essential component in winning poker. After all, if your opponents always knew what you had, they'd be tough to beat.
Deception is the art of keeping others off balance.
It's not the only way to bluff. The process is reversible too. Rather than acting strong with a weak hand, one can act weak when holding a powerhouse hand in order to lure opponents into a trap. There's also the semibluff. With a semibluff, as opposed to a bluff with a helpless hand, a player has two ways to win.