Studies and exams can be important for several reasons.
1. Retention of knowledge-- Studying and learning the material for your classes is important, because this knowledge will form the foundation for higher level skills and abilities later on.
2. Synthesis of material learned-- Many times exams represent the pinnacle of the learning process, the highest level of learning. The exam becomes the student's opportunity to bring all of the material they have learned together and process that information to combine all their knowledge on the subject into some sort of meaningful response, like an essay or research based test. The measure of your success or failure on an exam reveals your true retention and depth of understanding of the subject matter.