The explanation is that the movies famously cut out a LOT from the books, and fans were kinda sad about it, and the show aims to restore a whole bunch of that. In a vacuum, this would be a nice thing actually, like a Lord of the Rings show that restores the full content of all of Tolkien's books properly, the people want the Bombadil cut (oh look, what's Stephen Colbert doing over there?). Fans wanted it, and a lot of people will be very happy about it as long as it doesn't bomb. It's just a shame that Rowling will also be very happy about it, couldn't even wait for her to die so that they don't have to pay her.