This box allows you to recess the mounting arm into your wall. That means ~4" less protrusion into the room. Installation is pretty easy if you're handy with tools. The biggest issue is making sure that you have two studs that are 16" apart (standard) located where you want to mount your TV. So this is probably not so good for a retrofit, since the studs might not be in the right place. (Of course, you could open up the wall even more and frame out studs for it.)
It also has a knockout for a cable run in the bottom so you can get your wires for the TV signal (HDMI, Coax, etc) into the wall and a separate place for an outlet.
Only negative is that it's hard to install a standard outlet securely into the knockout.