Amazon tell us that the main reason for having the 3G connectivity is so that can browse the Kindle store and download their free and paid eBooks from just about anywhere the world.
Amazon do have incredibly good coverage on their 3G network.
So if you are simply away from home without an easily accessible Wi-Fi connection, or you are traveling abroad, you will always be able to access new eBooks to read. As a really avid reader or book lover that is an incredible benefit isn't it!
Another real benefit is that you can use the free 3G connection for other things than just getting hold of new Kindle eBooks.
You can use the experimental web browser and be able to check your emails on the web if you have no other way of accessing them.
Obviously the Kindle wasn't made for web browsing, which means that you may find the web browser a little slow, however if you have an emergency then it could be a real bonus to be able to access your email!
If you find yourself reading on your Kindle, when you are away from a Wi-Fi connection, and you want to quickly look up information the Free 3G connectivity is going to save the day. You might to find out more about a certain author on Wikipedia. With the 3G connection you can do that easily in many more locations than with the Wi-Fi only model.
Of course it's great that this connectivity is completely free for life! For the additional $50 more you have to pay you can avoid the need to be tied into a costly data plans, as you would with a device like the iPad 2. You can also connect from remote locations and indeed countries all around the world. Can you imagine doing that on your cell phone? It could quite easily cost a small fortune, but it's all included in the price of a 3G Kindle.