The Spanish had grown up knowing that gold could be swapped into land and wine and women. Get a lot of it, go home, live the good life. I doubt it was more sophisticated than that.Actually it really was. Spanish, Europeans, Asians, and perhaps other nations elsewhere, tried different methods of exchanging goods. In the beginning they would have exchanged good directly (salt for sheep), but they found this would be very hard because you could not readily divide certain goods and you had to buy significantly more or less of what you needed. Other forms of payments were introduced in different parts of the continents including salt, silk, and even nails in some regision, etc... Then came metal. It wasn't easy to divide, but soon enough they introduced small equally sized weighted metal. But as you can imagine the crooks would mixed different metals, which in the long run lead to coined money to certify authenticity. However Kings started to reduce the quantity of gold to repay debts they owed because it was not so much based on weight, but more so on "coin." There is more, but you would want to read the Origins of Wealth of Nations by A. Smith for more on the subject.
Gold seem to have been a good "currency," (but then again what do you mean by currency) and could have remain such, but the problem has always been the value it was based on and what you could have afforded with it. There layed the problem.
The above is what I understood from Adam's Smith Wealth of Nations, Book I, but I must admit I am still at my first read and perhaps by read 3 I will have a better understanding.