The DN-84254 is a microUSB cable which mirrors your smartphone screen on your PC. The sync software allows you to click around on the PC screen and control your phone.
So what’s it good for, imagine this:  If an app would require you to input letters or characters, you can simply do so by using the PC’s keyboard. Several key shortcut buttons on top of the program also allow you to conveniently make the device do specific commands, or access certain areas instantly. So, aside from being able to record screen activity directly on your PC (for gaming and reviews and the likes), you can also intuitively control the smartphone within the PC’s environment as if it was just a simple Android emulating program or similar.
For me, this cable is worth it just to enter contact info.
Unfortunately, we found this info on a Japanese website, so not sure how easy to get.
