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.
"One of the newest offered to market is Tazos..."
No Tazos are something different, does anyone proof read this stuff ?