Unless you're an absolute genius, chances are you'll need an outline to tackle a writing project of greater length. Note: Even if you are an absolute genius, it's still a good idea to organize those superior thoughts using an outline.
I'm currently gearing up for JuNo (National Novel Writing Month's June event), so I am rediscovering the value of a detailed outline. The book I will be writing for JuNo is called "Mindbreakers' Rebellion". It's about what would happen if telepaths started cropping up in distressingly large numbers. We're not talking Professor Xavier and Jean Grey... more like the gradual creation of a new species (tentatively called homo sapien telepathus, though my Latin is rudimentary at best. If anyone knows better, I'm open to suggestions on the genus and species naming conventions.)
Since I'll need to write at least 50,000 words for this story in the next month, I began writing an outline of the scenes. How to the two main characters meet? Which secondary characters are more important than they seem, and how can I drop hints? Is the villain hiding in plain sight, or hiding 'off-screen'?
By creating an outline, I can answer all of these questions before I ever put pen to paper. An outline also helps me determine what ending will be most enjoyable for the reader, most believable, and work out the challenges in writing whichever ending I decide on.
See the post above this one for prompts that need outlines. Have fun, and keep writing!