To be able to have the price of the product in multiple currencies. In addition, the price that is assigned is not the exchange of one currency to another, but rather a fixed one, the one assigned by the customer when creating the product.
Example: We have a product that we bill for 30 yen, but we also want to bill it at 30 dollars.
The ideal would be for rates, to be able to indicate the currency of the amount, so when applying that rate to the contact, they will be billed with the currency of the assigned rate.