This was long before my own kids were old enough for Harry Potter, but it gave me an advantage when they were. Originally I read to them, then them to me. Now they read faster without me, so we talk about books instead. It's a great series of books and movies, but the series gets darker and more intense as it goes along. The 3rd grader who loved the first movie, would probably find the last 3 pretty scary. I didn't let my daughter read Deathly Hallows until after she had read it. As a parent, I want to know what my kids are reading, what they are being exposed to, and what they think about it. By sharing an opinion about a character and the choices they make, we are also sharing something about our own beliefs. Which house do you belong in?
I also try to read ahead of their current level. It's such a good feeling to recommend a book, and have them love it. I have to be careful though not to give too much away. My daughter has reminded me (more than once) that I spoiled The Half Blood Prince for her by complaining about _____ murdering __________. They are pretty good readers for their age, so they've been walking that line between juvenile and young adult books. Sometime a book they are capable of reading, has themes that are to advanced. I had them wait another year before reading Hunger Games, and we watched the movie together and shut it of halfway through at her request. Other books had specific chapter I made a point to discuss as they read it. They don't always love what I love, but at least we speak the same language. Some families toss a ball, we read.
In a more general sense, reading popular YA novels gives some insight into the teen culture. Character references pop up in normal conversation. As a substitute teacher, I often pull current book or movie references into a lesson. Making a connection between a classic novel, and a current one helps them better understand it. Authors will often quote a classic in the chapter title, or the character will meantion it. A professor told me that there were no new ideas, I don't totally agree, but inspiration often does come from there. Even Barbie took her turn at playing Scrooge.