Its so you can connect say DVD Players, Xbox360, Xbox, PS2, PS3 etc to it to get 5.1 sound.
If you plan to use the above, its worth getting. Also if you plan to buy a HD DVD or Blu Ray Player, it will be useful so you can get 5.1 sound from them.
Many thanks. I will only have this sytem for about 12 months tops and it will stay in the house when I sell - I will spend more in the next property - so I guess the DAV500 will be adequate for now.