It's a subscription feature, just like the name suggests. Pay up-front, with no refund in order to get up to 20% discount. The discount applies during the whole lifetime of the server, just like prices will always be the same for already existing resources. However, as mentioned earlier, there are no refunds if you remove the server before the time runs out.
Up-front paid servers will also not be removed if your account runs out of funds, during the reserved period. Hence beside the discount it also brings predictability and protection.