I think Marriott did this to help members achieve their status quicker for those that stayed more than enough the year before. For example, if you stayed 100 nights the year before, you will get 25 nights rolled over to this year and it will count towards the "nights stayed" to earn the Silver/Gold/Platinum status. It takes 10 nights to make it to Silver, 50 for Gold and 75 for Platinum. So anything over those numbers will be rolled over.
I asked about this recently as well and the representative told me that this was a limited time thing so it might end in the near future. It just helps the members to stay less and reach the same status. So if we use the same example with 100 nights, it will mean you automatically have 25 nights this year. You will need just 50 more nights to remain as a Platinum member for staying over 75 nights the year prior.
Not sure if it was changed recently, but the Marriott website mentions 10 nights for Silver: