Save the risk of forgery, physical cash is still the only way for a seller to guarantee the dollar value of a transaction with an unknown buyer. Otherwise Paypal could block it, or your bank, or the buyer could reverse the charge, or whatever.

And that's why plenty of businesses still only take cash. Casinos want cash for chips. Peter Luger's still only takes cash. When you buy something like a dog from a breeder, it will be in cash. Any transaction where person on the other end has little recourse if your payment didn't go through will be in cash. Or if the business is popular enough to force it (in the case of Luger's).