In the beginning, BTC could be used to exchange of low value of virtual items, such as buying web sites or game points, virtual items, etc., afterwards, some people in order to get these virtual items, items may be in online auction own reality for BTC, others want to buy the real goods, but no BTC at hand, and then use real money to exchange BTC, as a result, BTC begin to flow, and have a "value".
In other words, bitcoin is inherently valuable as a means of payment; If it loses its function as a means of payment, it becomes worthless.
The value of money: it exists because of payment.
It can also exist as a store of value -- kind of like gold. Gold is impossible to use when it comes to buying small items like a pack of gum, but it can be used to make "paper money" that is backed by gold. So, bitcoin can be digital gold that is used to "back" other currencies, even dollars!