What I love about Harry Potter, is the HP universe. The original series, along with the Cursed Child and Fantastic Beasts builds such an amazing world. There are many different wizarding professions, locations, hobbies, and general intriguing ideas to look into. It's one of those world's that you could remove the main characters from, and people would still want to read it. Sure, Harry, Hermonie, and Ron are interesting, but they are mainly the plot drivers. JKR's amazing storytelling of Harry's life certainly is what sets the intrigue, but the world itself is fabulous and draws you in regardless of who's perspective you are experiencing it from.
Also, HP is one of those book series that don't end after you read all the books and watch the movies. As I said before, the whole world is very vibrant. There are many different interesting side characters you can still follow, and it's 100% open for OCs. In my opinion, some books (Like the Hunger Games) really need the main character to keep the series interesting. Sure, there's plot before the character was put into the driving seat, but it was basically just...boring. This is a good plot device, and really shows the need for the MC, but not very open for fanfics.
If you've already watched the movies, and need a reason for reading the books, I would say the extra bits are defiantly worth going over for. There were a lot of characters and moments that weren't in the movies, so those alone merit a read. Also, movies take place in 3rd person. The books aren't like that exactly. The narration indirectly expresses the character's feelings and thoughts about their circumstances. In the movies, you can't any of the character's thought processes, which detracts some in my opinion.
Anyways, that's my little essay.