Afaict, it's entirely a technical writeup about how to jailbreak. From the intro: "This document details the steps and thought processes I went through when developing the jailbreak. Ideally this will be a useful map or starting point for anyone looking at the system in the future".
I didn't find any information aimed at users of the Kindle about why they'd want to jailbreak.
I assume gp was talking about the 7th paragraph mentioning "whys" such as epub, fonts, screensavers:
>The mobileread community has done a great job developing extended functionality of the readers. Some of the more popular packages involve an open-source alternative reader with ePub support, a plugin to change fonts, and another addon to enable custom screensaver images.
I didn't find any information aimed at users of the Kindle about why they'd want to jailbreak.