Answer: I believe it's just custom. Some nations follow the quantity with a currency symbol, such as the dollar sign.
According to one hypothesis I've heard, the currency symbol was added before the quantity to stop individuals from modifying handwritten amounts, like changing 25.00 into 125.00, for example. It might be accurate, however I believe that the amount is always spelled out in words in the majority of situations when such fraud would be conceivable, such as on checks.
Explanation: