Harry Potter is all about Harry. He's our hero. He's the Boy Who Lived who is prophesied to defeat He Who Should Not be Named. So everything that happens in a Harry Potter book should be all about him, should in some way contribute directly to his hero's journey...right?
And yet, in Rowling's world, many of her characters, aside from her hero, have compelling goals and journeys of their own that only indirectly affect Harry's.
Fred and George's business ventures are a good example. There are points when their goals and Harry's intersect, such as when Harry gives them his Tri-Wizard gold as starter capital, or when they show Harry some of their dark arts defense products -- which Draco later uses against Harry and the Order. But by and large, the twins' joke shop and Harry's quest to end Voldemort are two separate journeys.
In Chamber of Secrets, Filch tries to use Kwikspell to improve his magical abilities. Lockhart gives advice to Hagrid on how to keep kelpies out of a well. Percy snogs Penelope in secret. And in Half-Blood Prince, Mrs. Weasley comforts Tonks on her relationship woes with Lupin.
