The USD used to be backed by gold, but they stopped that quite a while ago.
I understand that it does seem as bad as Compute4Cash. But at the end of the day, things only have value if there is a demand for them. Gold is only valuable because it has a limited supply and to many people it looks nice. The same with the USD, bitcoin or any other currency. (bitcoin being limited by technical measures, go read the source code)
I'm sorry you don't understand, and yes I realize it looks like a pyramid scam, and yes it could collapse at any point.
And yes, with cpu mining you get a tiny sum of bitcoins over time, gpu miners can make a bit more however. Also note that this rarity is part of what creates value (
http://en.wikipedia.org/wiki/Theory_of_value_(economics) "must be in limited supply")
In any case I've made my point and whether you agree, disagree or think it is ridiculous I don't really care. In any case understand what Compute4Cash actually is and that you could make around twice as much if you cut out the middleman.