This data empowers you with common questions such as:
Which activities have been skipped?
Did a student open the notes before doing the assignment?
Was the quiz attempted yet?
Did the student get a passing grade on the quiz?
Has the assignment been submitted?
These are easy to identify from the course page as they will now have a dotted checkbox that will automatically mark off once the student meets the completion requirements that you've set. You will have different completion requirements depending on the activity type. For example, the only option you have for a URL is making sure that the student clicks on it; however, for a Moodle Quiz, you can require the activity to be marked completion once the student submits the quiz, gets a passing grade, or has completed all available attempts.
Video Tutorial (12:15)
This video is very helpful in describing the different settings found with Activity Completion, suggestions for supporting those "click to view" activities, and details the settings for Restrict Access.