Cohen has stated, under oath, that he violated campaign laws "in coordination with, and at the direction of" the candidate. So now he is on record, under oath stating that Trump is guilty of conspiring to violate the law, and can be called to testify. Any testimony he gives has to be consistent with that statement, or a whole new criminal proceeding can be started for perjury (a new crime on top of what is now a criminal history).

This is better than cooperation in some ways because he is now on record as a witness, not as an informant.

Edit: Don't forget Cohen had an office in Trump Tower and worked almost exclusively for him - it will be hard for Trump to distance himself from Cohen.

BOOM!