To outline or not to outline; that seems to be the question today. To outline before you start writing can definitely be helpful; if anything, it keeps you organized. However, outline is a matter of taste and type of personality, and of “what works for you.”
Some authors, like John Grisham, make an extensive outline, before writing. He needs his outline (as told in an interview with Borders). His style is very disciplined and structured. He writes from 6-12 M, mostly 5-8+ pages a day. For others, outlines are shorter, or created along the way.
The important thing here is that outlines, extensive or short, are a good way to keep your thoughts organized, and helps in making the best of your writing time. Outlines are not permanent, they are just a guide to move you right along, but can change at any time. The story is alive, and sometimes it takes a path on its own.