These fixators can be very helpful when your ORIF of the articular surface is tenuous -- as in a comminuted fracture (especially in osteoporotic bone) fixed with multiple screws and pins. When you fear that it is not stable enough and are considering putting the patient in a cast to try to protect your fixation, consider instead a bridging ex fix, not only for the wrist and ankle but also for the supracondylar/intracondylar fracture of the distal humerus and distal femur with wimpy fixation. A bridging ex fix will protect your ORIF much better than a cast!
Ligamentotaxis alone often results in an imperfect reduction, as seen here where the dorsal and volar ligaments of the distal radius have tilted the articular fragments. Inserting a subchondral pin gives you a handle with which to tilt that fragment. If inserted a little deep (as shown), when the near fragment is tilted the far fragment can also be tilted.
After the fragments are tilted to look good on fluoro the pin is inserted into the far fragment to hold that alignment. Once pinned, some of the tension may be released from the fixator to prevent slow healing from overdistraction
Need more on plates, intramedullary nails, and external fixators.