WHY should I buy this coin?

You have to understand a fundamental principle regarding this coin. There is a difference between asset backing and being able to buy something.
The OP states that there will be silver coins made that can be bought on their website. I assume this means that the coins can be bought with BTC or Credit card. This method is not asset backing. On the other hand, if a coin is asset backed in the proper sense of the words it would mean that there is value that the coins can be exchanged for at a certain rate specified and that rate can be calculated at total value total coins. If is this guaranteed rate that then gives the coin a value.
If the dev's intention is that the silver coins can be bought with BTC or anything non-WBB, this means that the silver coins become just another commodity that can be purchased at whatever the price of that the coin is on the exchange and the coin goes in free float. If the dev quotes the silver coins at a fixed price in WBB without looking at the coin's price in relation to BTC, then it becomes closer to asset backing because whoever pays the WBB as quoted will get a silver coin that they can sell for fiat.
Before anyone jumps up and down with joy about silver coins, let the dev tell everyone how the silver coins will be used to peg the value of WBB.
Hi jc12345,
Well said and I could have not said it any better.
What I am able to tell you is, that the "
Silver Collector Coins" can only be acquired with WBB. The starting value will be set in WBB. BTC & CC cards can not be used to acquire the coins.
What will be shown also is a market value in CHF with conversion an estimate to USD.
More info will be released within the next 7 days (latest by the 07.03.2015 at 2100 CET)

On that date all will be announced what the next steps of WBB will be

Thank you all for your support so far.
Your WBB Dev