When you import: When you want to move class resources and activities from a class currently in Moodle (generally the current and within the past two academic years.
When you back up: At least twice a semester and at the end of the course. Keep only the newest copy. Backups are useful for classes you teach less than once a year. If you want to keep a back up for a future import (for content), be sure to download the backup file (.mbz) without opening it, to your computer.
When you restore: When a class is not currently in Moodle (the current and past two semesters) and you want to reuse course content from a backup.