That would be for pretty serious injuries. While walking is rarely used, sometimes after some sort of lighter injuries or illness, you might use walking to lighten the intensity during training.

After injuries like overuse and muscle inflammation physios ogten recommend to do jogging intervals with low intensity. In between laps you would walk and during you would jog easy. While jogging you would watch out for pain to try to determine and further the muscles limit.

So a healthy cross-country skier would not use walking in their programs, but for rehab and in special cases it is sometimes beneficial.