Do you have or are you getting the Canon EF 100mm f/2.8 L IS USM Lens? If so, definitely consider getting the Canon Tripod Mount Ring D as well.
The Canon Tripod Mount Ring D is shown above mounted to Canon EF 100mm f/2.8 L IS USM Lens. The 100mm f/2.8 L IS lens does not require this ring to be used on a tripod (the camera body tripod insert can be used), but the centrally-positioned Canon Tripod Mount Ring D provides a better tripod balance - and affords less movement after the tripod head is tightened.
Better-balance is not the only advantage. Since the tripod ring freely rotates, a nice amount of tripod setup flexibility is gained. With the Canon Tripod Mount Ring D in use, switching from horizontal to vertical orientation is fast and easy. Otherwise, a major tripod head adjustment is required (such as dropping a ballhead into the drop-notch). Correcting camera level is also done easily using the tripod collar.
The thumbscrew tightens/loosens the tripod ring friction as needed. Simply rotate the ring to the desired position and lock it tight. If not needed, rotate the ring to an out-of-the-way position when not in use. Alternatively, loosen the tripod ring locking knob and pull out on the thumbscrew to completely release the ring from the lens. This release is accomplished via a hinge (noticeable in the pics). The hinged design allows the Canon Tripod Mount Ring D to be removed without the need to unmount the lens from the camera.
A tripod ring allows various accessories to be attached to the lens - flash brackets being one of the most popular. I use a Wimberley P20 Lens Plate on my Canon Tripod Mount Ring D. This allows me to quickly mount the lens to any common Arca-Swiss-compatible quick-release clamp (Arca-Swiss, Wimberley, RRS ...) on a ballhead, monopod clamp, flash bracket ...
I do not consider the Canon Tripod Mount Ring D to be a smoothly functioning tripod ring - it is very similar to the Canon Tripod Mount Ring-A II in this regard. But the D's utility and good build quality is worth the price to me. And it looks cool.