[britta]

For most people it's about customizing iOS: adding extra features to the interface, disabling features you don't like, personalizing how it looks - generally finding ways to improve it so that it works better for you. Instead of just looking at interesting concept designs for new iOS features, you can install them. Here's a long thread from the jailbreaking subreddit where a bunch of people discussed the first packages they planned to install after jailbreaking: http://www.reddit.com/r/jailbreak/comments/17gm4g/since_this... - I think it provides a decent cross-section of some benefits that people get excited about.

Also, if people are interested in technical details about what the jailbreak actually does, this old HN thread with comments from saurik is very informative: http://news.ycombinator.com/item?id=4127801