Like others have replied already to: look around on the system, to transfer media (mp3, documents, etc.).
I also used it to develop and learn the Objective-C on iOS. I didn't have a Mac at the time and didn't really have to money to buy one either. Eventually this got me into a 3 month job (sabbatical from my regular job) to develop a prototype on the iPad for a startup.
Not quite, the iPad gig was on OS X using Xcode (ugh!).
My first experience developing on iOS was SSH-ing into my jailbroken iPhone. Here are my notes on setting up the development environment but these are old by now and very probably not relevant anymore: https://www.dropbox.com/s/naq7763aximu8cx/iphone.org
If you are really interested I have a tarball with source and Makefile for you but it won't compile on recent toolchains I guess. (Also a bit of googling will get you examples as well.)
This is all thanks to saurik (who replied elsewhere in this thread) and others in the jailbreak community.
Also a lot of apps on Cydia were written using the GCC toolchain (at least initially, I'm not really up-to-date anymore).
I also used it to develop and learn the Objective-C on iOS. I didn't have a Mac at the time and didn't really have to money to buy one either. Eventually this got me into a 3 month job (sabbatical from my regular job) to develop a prototype on the iPad for a startup.