The dog bone mount insert is a fairly cheap mod that stiffens up the stock mount. It reduces engine movement a small amount. On manual transmission cars, it is supposed to help provide crisper shifts. I can’t attest to this though as I have the DSG transmission. On a DSG car, it does reduce the rubber band effect that happens when you let off the throttle at higher rpms. Installation is pretty easy if you have a decent jack. Fitting the the insert requires a little force, maybe a little bit of lube. Use a rubber safe lubricant. I used the jack to shove it in. A plastic mallet may also work. If you are not looking to replace the mount itself with something stiffer, it’s worth the 30~40 bucks I spent. I bought it at ECS tuning. Full replacement mounts currently go for ~200 dollars. The golfmkv.com folks have found a way to diy for about 13 bucks.
I don’t recommend just purchasing the insert alone anymore. The BSH Pendulum Mount is a complete modification and does a lot more than just the insert alone. I’d advise you to go with the full pendulum/dog bone mount.