The purpose of a panoramic head is to allow you to accurately position your camera so that when you turn it you are turning it about the no-parallax-point of your lens. By rotating the camera around this point, you avoid parallax.
Panosauras Tripod Head
For those of you unfamiliar with Panoramic, QTVR, and Super Megapixel photography the need for a panoramic tripod head
may not be understood. A standard camera tripod is designed to hold and rotate a camera around the tripod mounting hole of the camera. This hole is located arbitrarily somewhere on the camera body. However, to take accurate panoramic photos it is important that the camera be mounted and then rotated around what is called the optical center (sometimes called the nodal point or
entrance pupil) of the camera's lens. This ensures that optical distortion, called parallax distortion, will not be introduced into your pictures before they are then stitched together using computer software.
The location of the no-parallax-point depends on the lens used, not the camera. Also, if you have a zoom lens, the position of the no-parallax-point changes as you zoom in and out.
All professionals use a panoramic head, except if it is simply impossible (because of the size, weight or whatever) to use one. Parallax1 can result in a poor quality picture, so if you are selling your pictures, printing them very large or just being a perfectionist, you'll want to make sure you have a panoramic head so you can get the best possible pictures to produce the best possible panoramas.
A panoramic head also makes shooting the pictures much easier, more systematic, and means that you will probably spend much less time trying to stitch the pictures together afterwards.