I believe purpose, and consistency are important because. At first you have to have a reason to right, a goal, in so many words. With this purpose you have overcome the biggest issue of what to write about. As I would say that is half the battle. With consistency, you need to be aware of what your intentions are. And the fact that, you do things the same way. Repetition can be an important ideal in writing. Without consistency, things may not be in order or confusing because of contradictions.