Deep snow hinders movement on foot.
By using sonwshoes, a person can regain the movement roughly equal to walking on ground. Skis on the other hand, offer mobility and speed exceeding what is possible on foot. However, both have their individual high points in various backcountry conditions. If there is more than one of you, then every 15 - 20 minutes, you should trade off who leads. If you are dogsledding, you should still trade positions from sled to trail breaker every 15 - 20 minutes, however there are some other considerations.