The key advantage of correspondence courses lies in the fact that you are not supposed to be at the place where lectures are being held. The lectures are planned by the institutions themselves and they send you the printed notes at your door step through courier or post. That saves a lot of your time when the institution is far away from your place, as you need not spend time traveling.
Another advantage is that while in the regular courses you need to take down notes for a particular lecture; here you do not have the need to take down the notes, as they are all available in the printed form and delivered straight to you. In the regular courses, the lectures are conducted at a fixed time. The correspondence courses allow you to have flexibility in terms of time, as some people prefer studying during the day and some prefer studying early in the morning, while some prefer studying late at night.
The correspondence courses allow you to access materials of different institutions at the same time, which can be a boon for students who are willing to refer different materials in order to score more in their respective exams.
This can be an added boost to the students who prefer scoring more in their exams rather than participating in extra curricular activities.
The correspondence courses can be accessed by anyone from anywhere in the world. This has made the world a smaller place in terms of connectivity. A person in India can access a course in the US or even Europe via correspondence. This can be beneficial to people who are talented but don't have the money to travel abroad. And this saving comes to a substantial amount, as the airplane tickets are becoming costlier.