Because, Mike Cohn's book, Agile Estimating and Planning is such a book that lays out the philosophy behind agile planning very beautifully, while leading the reader through day-to-day activities.1
And, as much as I tried to keep the review short, I did not succeed: There are so many nuggets of valuable information throughout the book that I justified making an elaborate review, and decided to publish the review in multiple parts. This post is the first of those.
If your agile development environment already has a set of tools predetermined, such as Rally Software or Jira Agile for example, your day-to-day approach to managing the development may already be somewhat predetermined by the software tool. However, whether you are in such a predetermined environment, or in a pristine environment with possibly no tools other than Microsoft Office, you'll need to recall the basics crisply, and Mike Cohn's book is one excellent reference source.