In short, Gradebook allows teachers and students to view grades on all assignments submitted and graded. For many types of quizzes and tests, this is automatic and grades are usually available for viewing immediately.
Workshop is one example of where this is typically not true. Gradebook can be of great use to instructors and students, but it will not do everything for you; it will not automatically grade everything, stop mistakes from happening, make students use it, or replace hardcopies of work. What it can do is help you with your grading and grade organization, allow you and your students quick and detailed access to grades, allow you to give feedback to students based on their scores, allow a quick review of students' or an entire class's grades to spot problems, and generally make grading a bit easier.
Gradebook can be used in several ways, but to get the most use out of it, we need to understand the basic options available and how they work.