Why is the half ounce of silver valuable to you?  You aren't going to put it in your teeth, and you probably don't manufacture mirrors or photographic film, so... it is probably valuable as nothing more than a means of trade.  Same with the bitcoin.

Why silver?  This is due to nothing more than a global consensus that it has all the properties needed for use as money.  Gold is chemically more stable, which makes it more valuable as money, but also is very difficult to use in small units, creating a demand that silver happens to fill.