Mechanically, it doesn't work for the reader to inhabit the head of, say, Dumbledore. First, Harry as outsider lets the author introduce new concepts like magic

Mechanically, it doesn't work for the reader to inhabit the head of, say, Dumbledore. First, Harry as outsider lets the author introduce new concepts like magic gently and slowly so it's relatable to the reader, and secondly part of the reason you choose first-person perspective is that it lets you keep the audience unaware of the big picture until you want them to see it.