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 ?
I highly recommend anyone curious of the space to research Ethereum. It is gaining traction for good reason. It's a bit like Bitcoin but you can run code called smart contracts on top of it, making it very clever. As a currency alone, transactions are done in seconds for cheap compared to Bitcoin which for the same price will take hours or you can pay very high fees for the transaction to go through quicker.