Right now if you are out for shopping or for any other errand which requires money at the end then you have a lot of different options to pay to the shopkeepers. Online money transfer is one of the best choices. But in the past, it was not the same case. That time cash was the only option. You were not able to send money online. So you had to be really careful to keep enough cash with yourself all the time.
Well now even if it is for cash still you can get it transferred anytime anywhere. And you must know that on a daily basis billions of US Dollars are being transferred around the world. Even if you want to send money to Pakistan, or send money to Nigeria, online options are good to go. And the reason why the dollar is being used as a currency for transfer is that it is one of the common currencies in the world, and being accepted at almost every single currency exchange.
Before there were just Cheques and bank drafts which were the common ways of cash deposit and withdrawal. But now you can have online banking, smartphone applications, debit card, credit card, and several other similar methods to get your money transferred. No doubt transferring cash in its physical form is a bit tough, so they are being transferred in the digital or electronic form.