These features of Moodle can be used to establish automated rules about how and when students can access some materials, or they can be used to give you a better idea of who has completed what on the site.  They can be useful if you want students to view or complete a particular item before they are allowed to move on to the next.  Or you can use them to give (and the student!) a clear idea of what they've completed and what they haven't on the site.  There's a lot of overlap between these two features, so it helps to come get the overview to see how you might be able to take advantage of these tools.