The strength of Potter fandom is what fans create on their own. They read the books and watch the movies, but that's just the start. They create their own fan fiction, their own costumes, and their own music.
In 2002, the DeGeorge brothers in Massachusetts started a band called "Harry and the Potters." Rowling could have shut them down for using the name, but she didn't. She knew that fan activity helps her by stirring up interest. The Potters were just the first of hundreds of Wizard Rock bands. There's Draco and the Malfoys, the Blibbering Humdingers, the Whomping Willows, and many others.
Wizard rock (or wrock for short) covers a variety of musical styles but specializes in songs about the Potterverse. The lyrics are important, not just something to hang a tune on. The songs comment on scenes and events in the stories. For instance, "Never Going to the Bathroom Again" tells us:
There's a faucet that leads to a monstrous snake
That will turn you into stone if you look at it
There's a girl that's been dead for fifty years
She lives in a toilet and has a crush on me
From 2007 to 2013 a small music festival called Wrockstock took place in Missouri. LeakyCon, one of the biggest Potter fan conventions, will have an impressive collection of wrock bands in 2016, including Harry and the Potters, Tonks and the Aurors, Tianna and the Cliffhangers, the Parselmouths, Draco and the Malfoys, the Whomping Willows, and the Mudbloods.
Songs created by and for fantasy and science fiction fandom are old tradition. As far back as the 1940s, people gathered at conventions after the formal program was over and sang "filk" songs about the literature, the conventions, and themselves. Filk conventions take place every year in the United States, Canada, and Europe. An occasional Potter-related song will turn up. Steve Macdonald has written a whole suite of Potter songs, including "Snitch-Ball Wizard," to tunes from the musical "Tommy."
Wrock is a more specialised niche, covering just Potter themes rather than fandom in general, but it's inspired other niches. Twilight rock, or twi-rock, is based on Stephanie Meyer's Twilight vampire novels and the movies based on them. Fans of The Hunger Games have "rockingjay." Star Trek fans have put on operas in the Klingon language. All these subgenres fall under what's often called "geek music."
It's amateur music. No one has made a living from it. In an age when people too often just listen to big-name bands, amateur music is a good thing. The word originally means "someone who loves." Amateurs make music because they love it and want to share it. The bands are as likely to perform in libraries and bookstores as in music clubs.
Sometimes they share it to benefit charities. The Harry Potter Alliance has used wizard rock and other fannish activities to raise over £12,000 for civilian victims of the 2007 war in Darfur. Other activities have aimed at promoting literacy.
Since the movies of Harry Potter and the Deathly Hallows, the wrock movement has been less active, but it's not dead. Harry and the Potters are still active. The celebrations at the publication of Harry Potter and the Cursed Child included wizard rock bands.
It shouldn't surprise anyone that music is such a big part of Potter fandom. Music is a kind of magic. It can lift people's spirits or calm them down. It can bring people together and unite their feelings. In the books, music calms Fluffy, the Sorting Hat introduces each incoming class with a song, and Dumbledore calls music "a magic beyond all we do here." Wizard rock spreads music's spells throughout Potter fandom.