To complete a real-world transaction in which the buyer / seller requires a traditional currency.
Plenty of trades take place solely in bitcoin.
Think: drugs, extortion, you get the idea.
I'm not sure I really understand your comment. You could just as easily ask the question the other way around. Using cash to buy something is a method of exchange, that's all. Buying a cryptocurrency with cash is no different than buying foreign currency with cash. The only measure of the value you could ascertain from either transaction would be the exchange rate.