One reason is that some students might want to travel to a warmer destination to complete their SCUBA training. We encourage our students to complete there training in our local water. It is often said that if you can dive between Cape May and Cape Cod, you can dive anywhere in the world. We believe this to be true, so if you learn here, you start your diving adventure at a level above the normal. If a student desires a tropical dive, we will happily assist them by helping them to find a shop and instructor that can accommodate them. They will need to pay for those dives at their destination to the instructor that finished the dive training.
Another reason we separate the Open Water dives from the main class is safety.
Sometimes when a new diver progresses through the the class, they will find out if diving is right for them. The majority of students have a great time and enjoy there experience, while some students come to the realization that diving is not something they want to continue with. In that rare case the student can "Opt Out" of the Open Water Training portion and not have paid for something that they are not willing to continue with. That way the student is not diving when they may not be comfortable just because "they paid for it".