Bitcoin works like a lottery ticket. IF it becomes the standard international currency, each bitcoin unit will be worth many many times its current value. So, even though the chance of success is very small, people gamble on it.
You could say bitcoin's value is backed by dreams of getting rich with a lot of luck.
This is an interesting view. Recently I'm researching that John Law's money creation scheme and he hold a similar view that money's value is decided by people's consensus and belief. People's belief used to be religion related, but now science and mathematics have become most of the people's belief
It is not necessary for bitcoin to become a standard international currency to make dream come true, just become a quality medium of saving is enough to raise its value constantly, since more and more savings from all over the planet will enter bitcoin system. Today, government can not fulfill their promise of pension payment, so many people will save for their retirement privately using bitcoin
If enough people accept bitcoin as a medium of payment, then its value will be established like fiat money. All it need to hold people's belief is the trust or credit rating, and in principle bitcoin has the highest credit rating among all currencies due to it is based on mathematics and networks