The use of the word ‘Discount’ is important in this model. In Salesforce, a discount is applied to the Sales Price of a product in order to reduce the total value of a line item.
The discount is limited and controlled by Salesforce to be in the range 0-100%. You cannot have a discount more than 100% or less than 0%.
Comparing Volume Prices with Volume Discounts, you can see that Discounts can only be used to lower a final price whereas Prices can be used to increase a final price.
Volume Discounts are also useful when working with multiple currencies. If you wish to offer a 10% discount for a product, that discount couple apply irrespective of currency. So, provided that you ware lowering the final price, then rather than creating a separate Sales Price in each currency, a single Discount could be applied irrespective of currency.
Volume Discounts will only ever reduce the final selling price of a product.