It’s a setup for the RICO case.

In order to prove RICO, you need to prove in court that crimes were committed and that they were part of an organized effort.

Having lower level actors plead guilty establishes that there were actual crimes that were committed. You can’t argue that something is not a crime after there’s a conviction.

Now add in the promise to point fingers in future trials, and you can connect the bosses to these proven crimes to establish the organized efforts.