Why did Bitcoin first start to gain economic value? I am really having difficulty answering this:


  Why does a bitcoin have an economic value, such that there are people willing to
  trade US$ (or any other traditional currency) for these virtual numbers?


What did the creator of Bitcoin do at the start to give it economic value?
Satoshi created a system that:


Allowed users to trust transactions without having to trust any single entity.
Opened it up so that anyone could participate and exchange computation power for Bitcoins.
Is designing with a fixed size (21 million Bitcoins), he created an incentive for users to get involved early while Bitcoins are relatively cheap to generate; there is less risk of future inflation reducing the value of early adopters.