One of the things Avid brakes are known for is their Tri-Align Caliper Positioning System, aka CPS. It's a simple bracket that does a great job of aligning the caliper with the rotor, regardless of how smartly the fork or frame manufacturer positioned their ISO mounts. No shims, no hours of fiddling and guessing and torquing down bolts only to find the positioning off by a few mm. Bolt these on a bit loose, move to the right position by hand, tighten, and the bike is ready to stop rolling on command.
You'd want these so you can replace damaged ones, or so you can change the rotor size and still use your old calipers. As has been discussed in many places, changing rotor size can change braking dynamics, which also means the brakes can be used on different bikes. Say you started with a XC bike and want to put your favorite brakes on a DH rig; change the rotor, change the bracket, and the brakes will work with the new bike.
We have CPS mounts for forks, for stays, for ISO brackets, for non-ISO posts, and, naturally, we have them for Boxxer forks. Use the pull-down menu to find the bracket that goes with your mount and your rotor diameter.If the name in the pull-down does not specifically say 'Post,' it will be an ISO mount, with the exception of the Boxxer adapters, which will be Boxxer-specific.
It does exactly what is supposed to, it adapts. I like the nice finished look of the avid adapters, they have a nice beadblasted smooth finish and seem to have a durable anodized finish.