The current hacks are mainly for those folk who are not satisfied with the Kindle features "out of the box"
In a nutshell:
1. The fonts hack allow you to use a wide variety of styles and levels of crispness to suit your individual reading preferences.
2. The screensaver hacks were to allow you to replace the faces of "dead authors" with anything you choose.
3. The USB networking hacks are for members out of Whispernet coverage to access their firmware to reset the time, etc.
With all hacks:
1. ALWAYS download the "un-install" file - you have to clean off all hacks when Amazon sends out firmware updates.
2. You install these at your own risk.
3. Check out the MR Wiki pages for lots more information -
https://wiki.mobileread.com/wiki/Category:HowTo