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.
I agree. It's onset and invention was geared to have an alternative or another option so-to-speak to pay for good and services. Although it has long since evolved into other users such as a means of investments, it can still be considered a currency because it possesses value in order to be used as how money is used at its most basic way. Having said, that characteristic makes it a currency together with its other uses.