Amazon Prime used to be an add-on service that would set you back just under $80 a year. With it, Amazon would provide you with free two-day shipping on millions of items, free streaming of thousands of movies and TV shows, and a free Kindle book to borrow each month. However, $80 is a bit steep to pay at once.
Not to mention, you may only want to be a Prime member during the summer when you have free time. Hulu and Netflix, which offer streaming just like Amazon Prime, offer their service at $8 a month. And now, so does Amazon. Amazon will allow Prime users the option to pay $80 a year or $8 a month.
Of course, the yearly option is cheaper, but only if you're going to use it for the full year. If you only need it for a month, it makes much more sense to buy the monthly option for a month. This could also be a great way for users to try an extended trial. Amazon already offers a free one month trial, but you can easily add additional months for $8 until you are sure of the product.