The settlement time. Gold and silver don't work well as currency if you need to settle over long distances quickly. So like I cannot send Amazon and ounce of silver to buy a product easily. But I can send them Monero with a few clicks of my keyboard. Almost instantaneously. Lynn Alden, who is an economist, basically says that the invention of the telegraph broke gold and silver as money, because transactions could happen at the speed of light, but settlement could still take weeks, especially if, say, the United States was paying, I don't know, the UK or something.