In the case of Bitcoin this is easily explained: Silk Road, "laundering" (for good or bad) and speculation. Since this is the first time many people have heard of this currency a natural question is: who is using this and why?
In the case of this specific currency, it seems like it's mainly used on social networks like Reddit for small transactions (tip), and due to its origins as a cheap joke currency it managed to get early traction. With this explanation in hand, the concrete uses becomes more clear.
Abstract values don't exist in a vacuum. There's always a concrete use of it, even if it's just speculation (Tulips) or gestures of goodwill (~ tip).