A grade book centralizes the grade items for your course. It's important to maintain a grade book so that students can stay updated on their progress in your course.
Weighted grades allow you to assign percentages (weights) to grade categories and/or individual items within Brightspace. You can place a higher value on certain categories of grades in the overall evaluation of students. They are more flexible:
You can add or remove an assessment in a category (like a discussion) without changing the syllabus
You can change the number of quiz questions (points) without changing Grades
You can change the point value of a rubric w/o changing Grades
An item can have points but have no weight (nice for paperwork)
See this Weighted Grades Calculator help sheet to learn how to calculate the weight of an item within a category and the score a student needs on a missing item in a category.
About bonus grade items (D2L website)
FAQs (D2L website)
Everything you could possibly want to know about Grades - Google Doc from Portland CC