Since the magnetometer offsets are not available through the MAVLink parameter interface (since they are an AP_Var vector) this little feature allows them to be reset from the CLI.  Useful if you somehow get bad offsets or if you change magnetometer.  If you have a bad set of large offset values I have seen issues with the nulling algorithm have trouble converging to the proper values.  I have never seen it have trouble converging from 0/0/0, so this could be a useful feature from time to time.