In courses with many modules and units, navigating to the next module or unit can be very cumbersome process since the user must navigate to the next logical module or unit starting from the top of the list. e.g. if a course has 34 modules, and a user just completed module 33, they have to:
a. remember the name of the module they were just in
b. scroll or swipe down to the next logical module in the list (for a screen reader user, this is 34 swipes)
Ideally, when the user hits the back button, focus will be on the module or unit they were just in so that the next one is not only on screen visually, but also has focus. This prevents forcing users to either scroll/swipe, or for screen reader users having to swipe through the entire list of modules and units.