Here is an answer I just posted containing two concrete examples for things that you can do with the jailbreak, but not without it.
There are many more examples, as twobob pointed out above. In short, it gives you full access to the device and allows you to customize it in whichever way you please. ... Yes, there is a reason why it's termed "jailbreak"
And yes, this is already (briefly) explained in the README.