I'm glad Shono98 started this discussion since it gives me as opportunity to share a few of my thoughts.
Because I enjoy it – I've always been an avid reader, and in the back of mind, I'd often wondered what it what it would be like to write. When I found the LSF last fall, I decided to give it a shot and have found that I enjoy writing immensely. The magic that seems to happen when I sit down and a story that was just a few vague ideas in my head starts to write itself is something that I never seem to grow tired of.
To become a better writer - I do a fair amount of technical writing for work, and I'm reasonably proficient with the basic mechanics of writing (grammar, punctuation, etc.), but I'm so new at fiction that continuing to write can only help me to get better at the more stylistic aspects of it.
To share - As far as I know, my wife is the only person that knows anything about my "kink," and being able to say and express things about my interest to like-minded people thorough my writing has been very liberating.
For feedback - I'm serious about wanting to become a better writer and feedback is a tremendous help in this regard. Before I submitted my first story on the LSF, I posted an earlier, considerably rougher version of it on another website. Although my story on the other site has been viewed over 9000 times, it's only received 7 comments! To top it off, very few of them are anywhere near as thoughtful or as constructive as the ones that the wonderful readers post on this site.
Ribbons – Having at least a mild case of OCD

, getting those colored-coded ribbons for accomplishments (number of views, number of comments, etc.) motivates me. I don't see ever getting the red ribbon for submitting 500 stories, but I have a big enough backlog of ideas for things that I'd like to write that I think getting red ribbons for all the other categories is "doable" given enough time.
To create a legacy – Not to be overly morbid, but none of us are going to be around forever, and the thought that something I created will be around to give others enjoyment when I'm gone (hopefully not for a long, long time) is somehow comforting.
LSF Publications – In addition to writing for the LSF free site, I've had the opportunity to write half a dozen spanking "shorts" (7000 – 8000 words) for LSF Publications. Nothing I've written so far is burning up the sales charts, so I won't be quitting my day job anytime soon, but I do appreciate Flopsy's patience with me and her sage advice about the commercial aspects of publishing (i.e., what sells and what doesn't). The editing work that Flopsy does has definitely improved everything that I've written, and I really enjoy not having to deal with mechanical aspects of getting my stories listed for sale on Amazon. Now that a collection of my "shorts" been compiled and listed for sale. I'm looking forward to tackling a longer form project like a novella in the 25,000 word range.
Fame and fortune - To be completely honest, the dream of making piles of money through writing is enticing, but I'm grounded well enough in reality to know that something that is highly improbable. On the other hand, the author of 50 Shades of Grey has a reported net worth approaching 100 million dollars, so apparently nothing in the world is impossible. I did recently read the first novel in the 50 Shades trilogy to get a sense of what the buzz was about, and although I could see why it might appeal to some folks, certain aspects of it grated on my nerves so badly that I can't imagine torturing myself with the second and third books.
In closing, I'd like to encourage anyone who's ever thought about writing to give it a shot and submit something for the site. If you've even remotely thought about writing any spanking fiction, I can't imagine anywhere more inviting than the LSF as a place to get started.