Imagine your book is a painting you’d like to complete.
Painting is difficult! Few people make a well done, interesting, painting with depth.
Sometimes people only seem to like paintings of already-famous painters so it can seem pointless to try to break-in.
NaNoWriMo is a time limit to make the pencil outline of your “painting”. At the end of the month, you’ll have 50,000 words (I currently have 32,702). Due to the time limit, there aren’t many spare minutes to use your eraser. Instead, you focus on completing the outline, knowing nothing is permanent. Then you take a step back and look. You might be pleasantly surprised. In December, you can make changes you think are necessary, then you begin to fill in the painting with color.
In the end, you may not recognize the original work you did during NaNoWriMo. However, you never would have finished the lovely painting in front of you if not for challenging yourself to complete your daily goals. 
Ashley Crookham's Writing 