Secrets of the FSB: Why do they hide archives?

The main Russian special service inherited the archives of the Cheka, NKVD and KGB - a huge mass of documents, to which many people have long been trying to gain access. Some write historical works, others try to rehabilitate relatives, others are interested in the fate of historical figures. And all of them encounter a reinforced concrete barrier behind which this information is hidden: many papers of the Soviet special services are not released even 100 years later, when none of the people mentioned in them are still alive. Why do they do this? And what do those who do receive something from the archives, other than formal replies, manage to discover?