This dock is quite useful as it means that you don't have to be continually plugging the USB2/firewire cable into the ipod, I always feel as if i'm going to damage something when I do that. If you use the dock, then one end of your USB2/firewire cable goes into the correct port on your PC and the other end goes into the dock. Thus, whenever you need to synch or recharge, you just drop the ipod into the dock. The dock also provides a line-out for connection to a stereo, which is useful because it's better quality than using the headphone jack.
However, £30 for a small unit that, when it comes down to it, isn't essential by any stretch of the imagination is quite simply a rip-off. My advice is, if you want one, then just buy the 40GB 4G ipod which comes with a dock included. It works out as much better value for money.