Let us look at it from two different perspectives: the Dive and the Instructor.
For the diver going beyond open water means continuing their diving education/training. The only true way to learn to dive is to dive. Under the supervision of an instructor, you will learn how to do it properly. You attain new skills and become a more confident diver. How far do you go? As far as you want. Instructors and most divers will say that rescue is a minimum. Open water class gives you the basic skill you need to dive, there is so much more to learn, and many ways to challenge yourself. It is worth the investment.
For the instructor it means promoting continuing education and teaching it so that the students have fun and learn at the same time. Let’s face it, if you are only teaching the open water class; you are good at kneeling on the bottom, demonstrating and evaluating mask clearing. As an instructor, continuing education classes give you a chance to practice your own skills and teach students how to really dive. You may be really good at clearing your mask but when was the last time you swam a search pattern or practiced your rescue breathing?