This accessory was a necessity for me when setting up the Harmony Smart Control. The package contains 2 cables (each with 4 small IR transmitters) that plug into the back of the Smart Control hub. The cables fan out into 4 individual wires (molded together like a ribbon cable) that can be separated and routed to each component that needs to be controlled. Also included are 8 rubber boots that you can place over the transmitters to keep out stray IR signals and prevent the IR signal from being received by other devices.
The Smart Control Hub and the included wired transmitter broadcast an IR signal in multiple directions and it can be difficult to find a place where all the A/V components can reliably receive a signal. It seems like the basic setup is more geared for components placed in a cabinet where the IR signals can reflect off the doors to the front of the components. My A/V setup is not in a cabinet and the only semi-reliable control I could get with this setup was when I was standing in front of the system, where the IR signal could reflect off of me. The cables in this accessory are the perfect solution - the IR transmitters (4 per cable) stick directly to the IR receiver of the A/V component, allowing precise control of the unit. I did have to use Google a couple of times to find the location of the IR receiver on some of my components - the XBOX was a tricky one - it is behind the Sync button. With these in place, all of my components are controlled reliably from the Smart Hub - even when I'm not standing in front of my system.
There are other online places that sell comparable cables, but I didn't find any that were considerably cheaper. In addition, most of the ones I saw had a 3.5mm plug instead of the 2.5mm plug required by the Smart Hub, so an adapter would need to be purchased. Since this accessory is direct from Logitech, you are also (more likely) guaranteed that the IR LEDs in the transmitters are matched to the voltage/current output by the Smart Hub and won't overload any internal components.