What are some advantages of learning on-campus that one does not get from learning online?
It’s the "unknown unknowns" that are covered on campus. Other people and casual discussions with classmates can highlight gaps in your knowledge that you didn’t even realise you had, or help you avoid mistakes you didn’t realise were possible. Even just listening to the tutor answer questions from others can help a lot. Tutors are not infallible even in terms of course content, and sometimes a simple question can point to a syllabus area that should be covered more in depth.
Posting in online forums in the knowledge that your rookie mistakes could be immortalised forever means that people can be slightly more reserved about posting a questionable question (or even a slightly questionable answer that may well get full marks in an exam, but the answerer is just a little unsure) and some distance learning course forums can quickly become ghost towns of hopelessly dated information.