When you prepay for 12 months (a year) of web hosting in advance this can result in savings. Depending on the web hosting provider they may provide a discount of 5-10% off the regular price. When prepaying in advance the only issue is if you want to cancel the service you may not be able to get a partial refund.
Prepay in advance with your web hosting provider = savings of 5-10% compared to paying every month for 12 months
Cancel web hosting services after prepaying for a year = 99% you won’t get a partial or any refund from your web host