I see the point of it as to more accurately bring the books to life, give us important moments and whole characters and storylines that didn't make it into the films, sometimes very important points such as who were the authors of the Marauders Map and their relationship to Harry were not included in the films! Some other stuff can be improved on like Harry and Ginny's love story being so unexciting in the films, and Dumbledore yelling at Harry which he would never do, a lot of Snape and Voldemort flashbacks were missed out. So much littler stuff like in the first book some of the challenges they had to get past at the end to find the Philosopher's stone were missed out. So for people who loved the books before the films existed, this is really exciting to see it all brought to life in a way that is way more expansive and truthful to the original stories!
RE the music I don't know how they will top the score used in the movies, it is iconic. I hope they can use it and build upon it.