Subscriptions make uninterrupted Premium access possible. When activating a subscription, you don't have to worry about purchasing periods or making payments. This results from the fact that payments are renewed automatically (for every subscription).
At any time, you can deactivate your subscription (e.g. you can purchase a month's subscription and deactivate it on the day of purchase which enables you to purchase only one month Premium access). This is a convenient solution for those who need constant Premium access.