The reason is, it sets the tone for the day. Whereas you hit driver on the upswing and catch irons and wedges with a downward blow, a 3-wood demands a neutral strike: You can't be steep or shallow. Some guys out here—Hunter Mahan is one—stand on the range and hit nothing but 3-woods off the deck until they start flushing it. That's when they know they're ready to head to the first tee.
I recently got a new 3-wood. It has more loft and a smaller head, so it feels like a 4-wood. I still rip it just as far off a tee peg, but the more playable design gives me more shots from the fairway.