This article addresses benefits of a 1-year or 3-year plan vs a monthly subscription.
If you sign up for a 1-year plan, you pay for 10 months up-front and get 2 months credit value for free. If you sign up for a 3-year plan, you will get 3 months per year for free (a total of 9 months). You are free to spend the credits whenever you want within the contract period. In contrast, the price of the monthly subscription will be higher per credit, and the purchased credits will elapse at the end of each month.