If I remember correctly, it has its roots in the RICO statutes which arose from FBI attempts to break up the mafia. The idea was to make it easier to prosecute people who did not actually pull the trigger but who were involved in planning and ordering of such crimes.

Now, a wiretap of a boss ordering a hit could result in the same sentence as the gunman.

Unfortunately it has been abused greatly, including drivers who never got out of the car sentenced to death for a 7/11 robbery gone bad, etc.

Further, as you can imagine, it gives the prosecution massive (many would say unfair) leverage over associates to a crime, given that they could face the same, full sentence if they don't cooperate. It's a major reason behind the enormous percentage of criminal charges not going to trial.