you can cash it into money, then use the money
Yes, in the Coinbase card example I posted the crypto is converted to fiat because that is what the seller’s bank requires. This is like using a debit card connected to your USD savings account to purchase something in Euro, your bank takes care of the conversion.
But BTC can also be used to make purchases directly. Anyone at all can choose to accept it in lieu of any other form of payment and they can leave it on BTC or convert it to whatever else, fiat included. People do that all the time every day. The twitter scammers converted BTC to XMR to wash it, then likely into Rubles or whatever they use where they are.
The “not backed by anything” is a tiresome argument. Your USD are backed by faith and credit, literally. If you buy a stock it is backed by nothing more than a market assigned value which is exactly what BTC is backed by.
