Bitcoins have value because they are useful as a form of money.
Bitcoin has the characteristics of money (durability, portability,
fungibility, scarcity, divisibility, and recognizability) based on the
properties of mathematics rather than relying on physical properties
(like gold and silver) or trust in central authorities (like fiat
currencies). In short, Bitcoin is backed by mathematics. With these
attributes, all that is required for a form of money to hold value is
trust and adoption. In the case of Bitcoin, this can be measured by its
growing base of users, merchants, and startups. As with all currency,
bitcoin's value comes only and directly from people willing to accept
them as payment.
Source : Bitcoin.org