The affirmative has an infinite amount of prep time before the round to prepare their case, they get to choose what is in it. The negative has the right to run topical counterplans. This is true for 2 reasons. The plan is the focus. All ground which is not the plan is rejected topical ground or non-topical ground available for the negative.
Topical counterplans test the intrinsicness of plan planks to affirmative solvency of the problem.
The exclusionary counterplan uniquely tests all affirmative advocacy by selecting a portion of the affirmative plan and excluding it from the counterplan text. I'm testing whether or not plank 2 is integral to solving the problem of juvenile crime. The DA proves that it isn't. The exclusionary CP tests plan plank. If Plank 2 is bad then you don't want to DO plank 2 and the cp gives you an alternative option.