It is also for Single Responsibility Principle.
I have read a coupe of posts that I couldn't agree with more:
Both of these articles talk about a topic I have been talking about for years. Single Responsibility Principle. It is the first of the SOLID principles. I would go one step further and say if you are not doing TDD, you are still not ruthless enough. Do everything you can to make you and your code better. Some say "Stay Thirsty", I say "Stay Ruthless".
Chris Parker's post is better than I could ever write. Of course, anyone who quotes King Crimson is pure awesome in my book!