I think you have the acronyms backwards but I'll take a stab at the question.
I am not associated at all with Juniper or any Juniper education provider, but I do have a masters degree and teach in an undergraduate college program in a humanities field. These two exams are testing different areas of learning.
The written exam is designed to find out if you have a breadth of knowledge about a subject. It tests you ability to cover a wide area and know details within each of the individual silos of required knowledge. We use these types of tests to make sure students have covered all four corners of required knowledge for full success in a subject.
Practical exams are the test of application of knowledge. We know from experience that there are some students that can learn all the theory but cannot practice the craft. By having practical exams we make sure that the student can apply the knowledge in the way we need them to practice. However, practical exams are by there very nature limited in scope. You cannot cover all four corners of the subject in anything approaching a reasonsalbe time frame. So we need to have the written basic knowledge exams to be sure the practice is standing on a firm foundation and not just specific knowledge of certain things.