Rollover nights allow you to carry over excess elite nights to the next year's balance. Something I liked! For example: Platinum Elite is minimum 75 nights in a given calander year, I stayed 88 nights in calendar year. In the past I was just a better platinum guy, but now, they carried over the excess: 13 nights into this year's balance making it even easier to acheive platinum this year! You don't use them per se but they help you maintane your staus when you go over stated number of nights.