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)
About bonus grade items (D2L website)
Brightspace Grading Recorded Session (38:21) - Session from CID
Everything you could possibly want to know about Grades - Google Doc from Portland CC