Usually when someone is cooperating, they are turning in someone else (such as others involved in the crime) or otherwise doing something to help with the damage they caused (e.g., spy describes in detail the info that he turned over to our enemy).
Is he simply describing in detail the documents that he illegally destroyed or could there be others involved in his crimes?
One more thing, why should he ever get a security clearance again? If he was a lower level person who had so outrageously and intentionally violated laws about handling secrets, he would never get a clearance again.
"One more thing, why should he ever get a security clearance again?"
He may never get his security clearance back again. As I understood the stories, this is not guaranteed; it is just the first opportunity he will have to request getting the clearance back.
If Berger asked for that as a part of the negotiation, his motivation would be pretty obvious . . . he would want to get the clearance back in time to be a National Security "player" in the presidential campaign of Hillary in 2008. However, if the three years was proposed by the prosecution side, a cynic would see it the same way, with the following proviso. He could be firmly turned downed for reinstatement in July of '08, and the news of that rejection could be leaked, which would put Hillary on the defensive for wanting to involve a convicted criminal and security risk in her administration.
Realistically, I don't think Sandy Berger has a snowball's chance in hell of ever legally eyeballing classified documents ever again, and that suits me just fine. What we are seeing from the sidelines is a game the prosecutors are playing to get as much cooperation out of him as they can get. They know Sandy Berger did this to cover Bill Clinton's tail. Those notes had significance.