Kaplan's objective has always been to expose the deep liability of precrime technology, and to make way for his own growing militia to take over. He envisions a kind of coup d'état in which precrime authority falls to the military intelligence of his personal army of loyal soldiers.
Kaplan wants to discredit the Precrime Police Department, of which Anderton is founder and current police commissioner. If Kaplan could somehow use precrime technology against Anderton; if he could manipulate Anderton in order to expose precrime technology as vulnerable and susceptible to error, then people would become suspicious of precrime law enforcement and its authorities. At that point, Kaplan could openly present his militia to the people as an alternative, and garner public support for an eventual overthrow of the Precrime Police Department.
Basically, the benefit in making an example of Anderton is that Kaplan would disrupt the majority's trust in Precrime technology, thus opening up space for a social destabilization of precrime policing authority. This destabliziation would bring Kaplan one step closer to his ultimate objective: to establish statewide militia law under his direct totalitarian rule.