A screensaver is a program that displays images, animations, or patterns on a computer screen when it is idle for a certain period of time. Its purpose is to prevent the screen from getting damaged and save power. Full definition
The reason: by opting for a lower price, the user allows Amazon to push sponsored screensavers on to the device. (theguardian.com)
Or if you don't feel like reading, you might enjoy downloading Amazon's new interactive Admash that allows you to vote on sponsored screensavers for Kindle. (kindlenationdaily.com)
Special Offers, by the way, are sponsored content that appear as screensavers when an Amazon device is in sleep mode. (techradar.com)