The course end date is used for determining whether a course should be included in a user's list of courses. When the end date is past, the course is no longer listed in the navigation block/drawer and is listed as past in the Course overview on students' dashboards.
The course end date is also used for calculating the retention period before data is deleted. It may also be used by a custom report e.g. reporting activity for the duration of a course.
Users can still enter the course after the end date; in other words, the date does not restrict access.