The act of reading is a skill that needs maintenance. It doesn't matter much whether it's fiction or nonfiction.
Harry Potter is a bit of an unfair example as it's intended as a children's book, they still need to build up the skill.
But more importantly, good fiction (and other art) _teaches about the human condition_. I have learned a lot from Vonnegut and Pratchett even though everything I read by them was fiction.
I couldn't disagree more. Good fiction is more than just entertainment, and you learn as much from it than from a good non-fiction book. Some aestheticians have talked about how you learn from art the same way you learn from more obvious sources. Some non-fiction books are great, but there are a lot of non-fiction books that are just entertainment.
It's not the shelf of the bookshop on which you find it that tells you if you can learn something from it.
Reading any of "The Classics" is much more than entertainment, it is living another life with a depth you'll never achieve on you own. Very worth one's time to push mature ideas and contemplation into your head.
If your're reading fiction, then it is most likely just entertainment. So, its equivalent to watching a fiction movie or tv show, or whatnot.
I find it odd thats its encouraged and considered good to just read, regardless of content. What you're reading is critical.
You can read 'Harry Potter' and divert and entertain yourself, or read 'A Brief History of Time', and do the same, but also learn a bit.