While I agree that it's up to each writer to discover their method and the majority in this thread are pantsers, I prefer to outline using Word's Outline view. This gives me a skeleton of the elements of the plot, which can be easily changed, or new ideas incorporated, along the way. I can only "pants" flash fiction, even with that, I find myself running out of inspiration after about 500 words. I need direction from a pre-worked out plot and characters.
If you don't outline, how do you cope with things like research? For example, one of the ideas I have for this year's nano is set around 1920, in the English countryside, and one of my main characters is a woman around 17 years old who is literate. She's from a poor family, her father is a farm worker. So I have to research the education system of the time. I remember the "we must educate our masters" quote after one of the increases in voting rights, but I don't think there was free education in the UK at that time. And for a woman?
Stuff like this needs planning in order to write a realistic novel. How would you manage that without an outline? For me the outline provides direction.
I don't over-outline, just work out the basic plot steps. Word's outline view allows me to paste in the writing I do, and move things around. Also to get rid of steps I decide I don't want and include new ideas. For me, it's part of the fun of constructing the novel. I'll be doing an outline for my nano novel when I've finally decided which story to run with.
But different methods work for different people.