Well traders give steem value because they want it. Same goes for bitcoin. You can sell steem for bitcoin and other coins the same way you can trade oil for gold for example. You do this at an exchange. If your planning on going to cash you trade your steem for bitcoin. After you have bitcoin you can sell it for cash at exchanges as well. Once you have the cash on an exchange you just withdraw it to your bank account. That's all there is to it.