when we already have crypto currencies, master cards, paypal and other shit, why do we use cash?
Cash is good for medium sized transactions in person. If you were to go to a hot dog vendor for example cash is likely the best way to pay for your food. The payment phase of buying a hotdog is very short, only ~30 seconds, less then the time it would take to show a QR code, scan it, broadcast it and the vendor seeing it register as a 0/unconfirmed tx on the network.
Cash also provides a greater level of anonymity then BTC as it is generally not feasible to track where individual dollar bills have been in the past.
BTC is good for smaller transactions when the time between when payment is made and when the product is delivered is greater. For example when you use BTC to buy a coffee from a coffee shop it will take some time to give the customer the coffee from the time that payment is received. So the process will be: Give customer QR code to pay, customer scans QR code, and broadcasts transaction to pay for coffee, vendor prepares coffee while verifying the 0/unconfirmed has been broadcast, vendor delivers coffee to customer at their table.
BTC is also good for smaller, high margin purchases. The above coffee transaction costs the vendor very little to create the coffee so they would lose very little in a double spend attack.
