As the description points out, this is not a seat post collar. It clamps to your seat post and provides you with seat stay rack mounts. Great for small bike frames or bikes with no threaded seat stay braze-ons. I put one on my wife's XS Cannondale Quick 4 for her Topeak Explorer rack. The rack was previously mounted to the seat stays with the bracket arms stretched out to the max and bent downward when mounted to the seat stays. With this, I was able to shorten and straighten the bracket arms. It went from being wobbly under load to rock solid with this post-lock. We have no issue loading down the rack with 35-40lbs of gear or groceries, now.
If you install one of these, it renders a quick-release seat post collar useless because your rack will essentially be mounted to your seat post. Not a "con" in my book as it makes the seatpost harder for someone to steal, but it's something to consider if you need to remove your saddle often.