People who don't believe in Bitcoin and don't understand the fundamentals of a "A Peer-to-Peer Electronic Cash System" usually ask:
"What's the underlying value of Bitcoin?"
And the simple answer to this, is:
There is no underlying value in Bitcoin. Bitcoin is a tool for value exchange.
And to prove this, I am selling some of my artwork for cryptocurrencies (for example: https://steemit.com/art/@nandor/art-for-crypto-where-is-waldo-vi), and if I can sell it, instead of exchanging that cryptocurrency for fiat currency, I will use it to exchange it for value, and post about it here on Steemit.
Because fiat currencies have got no value, they are just a tool for exchanging value as well.
Gold has no value (besides it's industrial use), the imagined value of gold is just a part of our collective hallucination.
The US Dollar has no value, it's the soft- and hard power that the US Government presses on the rest of the world that makes the USD more valuable than the Mongol Tugrik .