Backup your Games
The primary reason for jailbreaking any device is to install cracked applications. But it can also be used to backup your games. For instance, if you're a bit clumsy and have the habit of lending your games to careless friends, this feature comes in really handy as it allows you to back up the entire game onto your hard drive so you can play it whenever without the need of a disc. You will lose online play but for games without a multiplayer features like God of War, it won't make a huge difference.
Save a lot of Dough
The next obvious pro would have to be all the money you'll save especially since console games aren't exactly cheap. Games are available on the Internet if you know where to look. After that, it's just a matter of transferring the games into the PS3and playing them.
Homebrew Apps and Games
A lot of developers have released custom applications also know as homebrew apps that extend the functionality of the PS3. For instance, there are PS One and Nintendo emulators available that let you play those games on the console. An app called Multiman allows you to backup your PS3 games onto your hard drive where as BlackBox is very useful FTP server that let's you quickly transfer files into the PS3 without a portable hard drive.