They add a little gold for a more stronger foundation of some items like the diamonds. The same is the usage of btc but in the cryptoworld.
Just think of it bitcoin must be compared to gold in order to fathom how it works. It's like a treasure and usage in real life is to be traded for it to have a sense. It can be used to buy things you want and most of them can be found in internet. Transactions over the internet is the real life usage of bitcoin.