Here is a well written concise list on the benefits of tethered shooting
1. A Large LCD monitor is a much better viewing medium than the small screen on the back of the camera. A vertically oriented image viewed full screen on a 23″ Apple Cinema display is the equivalent of an instant 11×14″ print.
2. Immediate feedback of focus, exposure and color. This is hard to see on the 2″ camera LCD with the limited set of processing tools provided in camera.
3. Speed is increased when you are writing data to large fast hard drives in the computer instead of to small CF cards. The camera buffer isn’t filled as often because the data travels faster and takes the load off of the camera’s internal processor.
4. Instant automatic backups using RAID or scripting. Mirrored drives (RAID 1), three drives stripped into one (RAID 3 or 5), OS scripting and utilities like Synchronize Pro X all provide a way to automatically backup your images as they are written to a hard drive.
5. Instant processing. The images can be processed immediately and delivered to the client before they leave the shoot. No waiting on 20 CF cards to load before exposure/color adjustments can even start.
These five advantages to shooting tethered all provide an amazing client experience. The client sees what is being shot and is better able to collaborate with the photographer. Providing the tools for this level of service creates happy clients.
Photographers who shoot tethered realize these advantages every day. Photography is a very competitive field and shooting tethered provides an extra level of service that the clients will remember. The next time a job comes up and the client is choosing between two photographers who do you think they will pick? The photographer who shows them 2 inch previews on the back of their camera, or the one who provides a seamless shooting experience with instant viewing and editing capabilities?
Source: PDN Gear Guide