Originals are challenging, because you’ve got to create the world, you’ve got to introduce characters. And in order to do that, you need to create them yourself – and once created, to simply scrap and move onto another project is...well, off putting. Writing up oneshots in the same world help with development, backstory. This is all stuff fanfiction doesn’t necessarily need. However, having delved into originals, when I came back to my fanfiction, I’ve been able to drum up some interesting new characters.
For my current potter fanfic, it’s uses GoF as canon (because this story predates the release of OotP), but it covers Harry’s years six & seven. Having original experience on this rewrite, I wanted a foil for Harry helping Oliver Wood with flying lessons for first years. So, I created this boy with issues, introverted, shy, and smart, but doesn’t really know his own father (solicitor trying to get out of his moral obligations), so that Harry spends the entire lesson helping this one boy rather than helping Oliver with any of the other students. Ash, the boy, kept poking his head back into the story, and has become an invaluable insight with a better Hogwarts perspective than I could in the previous iterations of the story, as Harry’s progressively more and more isolated from both Hogwarts and the wizarding world. Overall, Ash was a “happy little accident” that’s made the story better.