One of the primary things is that one doesn't know what is authoritative, and what is just some random kooks ramblings. In theory a certified course, from known academics (say), gets over that hurdle. It also allows you to put something on a CV (ie resumé), and allows you to check reviews first if it's a proper course (to measure how decent the course materials will be for you). If you don't follow a proper course you don't know what you missed; learning is improved with expert guidance of the pupil.