A JP1 cable will let you make your remote do just about anything you want. On the very basic level, you can use it to let you do ordinary stuff, like programming macros, etc from your PC rather than on the remote itself.

On the next level, you can add codes for devices that the remote can't control.

On a more advanced level, you can use it to do all sorts of cool stuff. For example, you could combine the codes from several different devices into one upgrade, kinda like how the "Home Theater" mode works in some of the newer remotes. You can also use it to get around any stupid restrictions that are built into the remote.

All in all, it's gotta be the best investment you can possibly make for the $14 starting price.