Sure and Bit Torrent isn't mainly used for sharing copyrighted files.
Hard to know what the main reason is, but it certainly seems to be the main reason on the jailbreak models advertised for sale.
How many really good unapproved apps are there that really make this a big deal for legit reasons?
That sounds like someone whose only knowledge of Jailbreaking comes from alarmist articles.
Besides Emulators, which are questionable.
Playback of non-iTunes video formats.
Video Recorders which are better on older iPhones than the ones offered in the App Store
Themes for icons
Themes for the entire OS
One swipe access to toggles such as wifi, 3g, ssh, auto rotation, etc.
Ability to hide unwanted apps.
Ability to free up RAM.
Various Lock Screen mods that display useful info.
Small system hacks that change certain functionality
Running background processes.
Task switchers for toggling between running processes.
Unrestricted app usage on 3G.
Tethering.
Creating Wifi Hotpspots.
Unlocking for other carriers (which is debateably legit but not piracy)
Various launchers and springboard organizers.
Controlling when and how you access your iPod controls.
Copy & Paste long before Apple provided (no longer neede but an example of a legit JB app)
Support for bluetooth keyboards
Safari downloading, which has the side benefit of opening links in new tabs.
Robust mail attachment support
File system access, and storage as an external drive
A more robust copy & paste pop up menu
Replying to Text Message from within the pop-up dialog which eliminates the need to quit what you are doing.
Dashboard-style widgets.
Creating ringtones from songs you already own.
A more robust Calendar App.
A firewall
Modifications to spotlight that search the web and specific locations.
Haptic feedback
Better management of you recent calls and voicemail.
Google Voice
This list is by no means complete, but as you can see, the majority of useful JB apps center around increasing your phone's functionality, not piracy.
Since the AppStore doesn't allow demos, one of the legit uses for the piracy app is to try apps out first. Assuming you then buy the app if you keep it.
PS. I've been JBing from day one and have zero pirated apps.