Think about it: if you are in high school and are taking AP Calculus or AP British Lit, and working at a college level anyway, why not earn college credit while you are doing it? The advantage to students (and parents) is numerous: these classes cost a fraction (about 10%) of what they cost if you are enrolled solely in college (it's an initiative sponsored and underwritten by the state, so costs are subsidized, and free to students who meet criteria of being first generation, on free and reduced lunch, low income, etc), the credits transfer freely (at least to in-state public colleges and universities), you are in a smaller section with a teacher you know who is in the business of helping students - as opposed to the near universal freshman college experience of sitting in a class of hundreds of students, often in a course taught by a grad student, and the instructor has office hours once a week - if you can get in- and expects you to already know how to write an essay and isn't about to re-teach the skill if you don't.
I'm just about to wind up my first year of teaching these courses, and have (so far) enjoyed the situation immensely. Good students, easy to work with, fun to teach. I get a second paycheck from the university that sponsors the classes (and I earn every penny of it, as I now have dual sets of bosses: double the pleasure, double the fun ! twice as many meetings, grades to turn in , paperwork to fill out, emails to answer, requirements of all sorts.) and I need, it, as I am putting two of my own through college at once. I'm hoping to ride this situation till I retire...and thus avoid much of the misery currently tormenting my teaching comrades (gigantic classes, no textbooks, more and more state mandated testing that takes up half the school year....but I digress. That's another topic altogether.)