Strictly speaking, I don't have to wear one at work—I'm a few years away from retirement; I've risen about as high as I'm going to in the organization, and as the in-house art department I rarely have to deal in person with folks outside the outfit—but I always have, even 20 years ago (before I landed the graphics gig) when I used to wear Levis to work, greatly to the irritation of my then-PHB. I outgrew the Levis (in more senses than one, alas), but it seems to me that the necktie serves two useful purposes: it serves as an acknowledgment by the wearer to whom it may concern that he serves within a dominance heirarchy and is prepared to abide by even its most arbitrary and pointless conventions, and the slight strangling sensation reminds this wearer, at any rate, that he's on hostile ground, and should not by word or deed betray to the heirarchy either resentment or contempt.
(Actually, I'm blessedly insulated from most of the office politics, regarded by the younger employees as a harmless eccentric and by management as a cross between a "computer whiz"—which I am obviously not remotely, but these are PHBs—and a court jester. Better a necktie than bells and motley, I always say)
cordially,