Dialogue and narrative will carry a story only so far. Many fanfictions on the 'net are merely a slice in the life of a particular character. Plotting helps keep the writer motivated during the actual writing process. Here's how plotting helps a story:
1. It ensures that the story/fanfiction/novel has balance. There is a beginning, a middle, and an end. Have you ever read a story that seemed to stop abruptly, as though the author ran out of things to say? A guideline for plotting is that the beginning should encompass 25% of the story, the middle 50% and the end 25%.
2. Plotting helps to avoid the "what happens now?" scenario, when the author realizes (as above) that he or she has run out of things to say.
3. Plotting keeps transitions smooth. This mainly applies to novel-length fiction with main storyline, subplots, and multiple POV characters.
4. Plotting helps keep the pace of the story consistent.
You can plot your story any way you like: index cards, spreadsheet programs, Word documents, etc. You may wish to subcategorize your plot and subplot and changes in POV. It's also helpful to have a timeline. If your story takes place over many years or seasons, get out a calendar and plot out the storyline.
Character sheets (or style sheets) help the writer to remember key details about a character. You don't want to write about a black-haired hero in the beginning and 50 pages later give him brown hair.
Here's an example of a character sheet:
- about 5'5' tall
- jet-black hair and dark eyes
- grew up in Pallet Town
- won the Orange Islands Championship
- started his journey with a Pikachu
Think of plotting as a rehearsal. You get an overview of your story so that you can weed out any obstacles or take control of subplots that dominate chapters. You can understand your character's motivations better as well.
[EDIT: Feel free to discuss. This is one of a few writing essays I'll be posting in the coming weeks.]