The point is, why would I gave my car for a grain of sand?
Why will you give your car for a million dollars in cash? Because people accept it as a currency. Because, you'll afterwards go to another person who's willing to trade their goods for your dollars. Simple economics where societies develop mediums of exchange.
Same happens with Bitcoin. It meets the conditions to be considered currency. It was designed by humans only for that purpose. What's so hard to understand about that?
Because with a million dollars, either cash or digital, I get the ability to recive capital at the dollar withdrawal from the market, which is at loans repayments/liquidations. Dollar is a security. Securities are shares in capital that the security issues borrowed and have the liability to return. Bitcoin on the other hand is not security. It is literally a number. Hence the question.