1. Fanfictions (or multiple fanfictions) can be just as long as a novel.

A large amount of the fanfictions I've read over the years have been as long as a full-length novel. While the minimum goal of NaNoWriMo is 50,000 words, fanfiction can have the potential to go above and reach 100,000 words or more. This also applies to one-shots since some are able to be expanded beyond their original premise. A not fully formed novel idea may leave you struggling to reach the minimum. However, a fully formed novel idea can leave you achieving more than you bought you could.

2. Fanfiction writing can take as much planning as a novel.

If you think that fanfiction required planning, that's far from the truth. It may already be in an established universe with established characters, but there are other things that require planning. For one, the plot requires planning time. It has to be outlined just as would with a novel. There are also times when one introduces an original character into the story. The original character is a creation of your own and requires a bit of outlining in regards to their role in the story and interaction with already established characters. In addition, alternate universe/scenario stories are the fanfictions that can cause you to dive into a bit of research. Let's say, you're going to place modern characters in the setting of the 1950s. It's going to lead to you researching the time period. This way you're still practicing the skills of planning and research, while also getting to chance to write fanfiction.

3. The universe and characters of the story are already created.

This reason here is what gives a slight advantage to fanfiction writing. As discussed in the previous point, fanfiction requires planning. Even though you may not be creating characters or the verse, you still have to plan to remain in it. If you know fanfiction well enough, you'll know that doing OOC (out of character) writing is an essential no-no. The reader is still expecting their character to remain the same, just seeing them in a different story/scenario. The establishments can seem like a slight advantage, but it's also the thing that makes it hard. Although, it does provide a challenge that can make your writing stronger in the future.

4. Fanfiction can give you instant feedback.

If there's one thing we could use, it's a confidence boost. The majority of the time when someone writes a novel, they work to have that novel published (and yes, sometimes it's online). From there, you have to wait for a reader to find your novel and then receive feedback. However, when writing fanfictions you typically post them online when finished or by chapter (update). Readers find them instantly by searching for stories in the tag and if you're lucky, leave comments and kudos. Of course, while there are negative comments, the majority can be positive. It's these comments that can improve your skills as a writer, especially if they come in the form of a good critique. Also, other fanfiction writers are willing to provide feedback in the form of a beta.

5. It gives you time to plan original material.

While working on another project, it can begin to help flow the creative process of another one.The fanfiction can help to influence the original material. The thing you may have been stuck on during planning may finally be solved while writing about another character or story. It's true that the characters we already know can subconsciously affect our own original characters.

In the meanwhile, you get to achieve some writing and a get a start on the next project.