Why might I need to adjust these dates?

Although your course is available to students as soon as it is published, before the start date and after the end date, students can only "see" but not "do." For example, they can see your homepage and read the syllabus but can't participate in a discussion. In other words, a published course that has not yet started or has already concluded is in a "read-only" state.

Adjust the start date (sooner) if you want your students to:

Complete orientation activities before the course starts; for example,

submit information, such as a practice assignment
take a syllabus quiz
participate in an introductions discussion

Adjust the end date if you want to:

Prevent students from submitting assignments or taking tests after a date/time that is before the actual semester end date/time

Additional resources:

Adjusting start and end dates - Canvas guide