I posted earlier this week about what I think of fanfic. I promised then that I would talk about the edge fanfic writers have over their literary peers. So, here's my list of 5 ways fanfic can help you be a better author. Please bear in mind, I'm not saying that fanfic writers aren't "real" writers, only that it can lend an edge to those seeking traditional publishing routes.
1. You learn to take criticism. Within the fanfic community, there's a system of receiving and giving feedback. Most are paired up with a beta reader, or they have a whole slew of them. The betas will help make it a better story. Additionally, when you post a fanfic online, people will post their responses. This is good preparation for changing a story in response to editorial comments.
However, the fanfic community also tends to foster more of a positive attitude than the reading public in general. Those coming from a fanfic background may react more strongly to a negative review than one elsewhere, because they've become accustomed to comments directed at them. Some have surmised that the recent backlashes against reviewers has come about from those "graduating" from fanfic. I don't think that's a fair assessment, but I do think that fanfic writers need to remember that not everyone will be as nice as the people leaving comments on your fics.
2. You learn to give constructive criticism. As I mentioned above, fanfic writers are frequently paired with beta readers. That means that they, too, must serve as a beta reader, and give useful feedback without upsetting the person they're critiquing. This is a good skill to have, and often works to make the people offering critique better at spotting problems in their own works.
3. You can experiment. Fanfic writers are working in someone else's world, with someone else's characters. That leaves them free to break any and every other rule, and to see how it works. It might not make for a readable fic, but it does make for a fun diversion for the writer. It also lets them think outside established literary techniques to find something else that works.
4. You get immediate feedback. If a fanfic writer is normally getting a dozen or so comments per installment, and suddenly that writer gets 100, that indicates she's doing something right. If she normally gets hundreds of comments but the response to her latest is only a dozen or so unenthusiastic remarks, clearly there was some misstep. It might not always be logical or obvious, but, if a pattern emerges, that writer can quickly figure out what her weaknesses might be, and work on them.
5. It's fun. Writers will tell you how writing is hard work, it's a slog, and it's not always fun and games. But, if they didn't enjoy some part of it, they wouldn't do it. There are plenty of terrible, joyless, difficult jobs that pay more. Writers write because it's fun. If you're having fun while you're learning the ropes, then you're well ahead of your degree-earning peers, who are slogging their way through classes meant to teach them how hard writing is.
If you're a fanfic writer, you may not benefit from any of these. You may not want to. But, if you are interested in traditional publication on an original work, all of your writing practice may serve you well, for the above reasons.
I would advise against submitting your fanfic for publication, though, for various reasons. Most of the spinoff series you see on the shelves were specifically commissioned by the publisher by authors who were already established as publishable, by their own original works. And don't approach the authors themselves; they'll refuse to look at your fanfic for legal reasons.
Instead, you're best off sharing your fanfic with the fanfic community, and working on your original fiction on the side as you hone your skills. Nothing says you can't borrow elements after you file the serial numbers off, but there is a difference between an element here and there and a recognizable story and characters.
There is one notable exception, and I won't invoke its name. I'm sure you all know what I'm talking about.