Having a real life teacher while practicing Chan is nearly always better than not having one. However, practicing chan with no real life teacher is nearly always better than not practicing it at all.
You could think of it like a university course on some academic subject. (Even though chan is not an academic matter) Can you just buy the book and learn all the class materiel yourself, do the lab experiments yourself, etc? Sure, you can. Does it help to have a university professor teach you the materiel, explain things in the book you are not clear about, answer your questions, give you homework and hold you accountable for completing it? Of course! That is very helpful!