One of the aspects of the Little House books that confuses modern day readers is why Jack always trotted along beside the covered wagon instead of inside it, including when they had to make dangerous water crossings across fast moving rivers.
During the era that the Little House books took place, dogs were considered less pets and more a working animal on a farm, similar to how other animals such as horses or cows are working animals. And for pioneer families, especially those travelling west, a dog was seen as a necessity to provide protection to the family.
Dogs could alert families to any dangerous animals or anyone approaching their campsite each night, which is also why Jack slept under the wagon instead of within the wagon box with Laura, her mother and her sisters.
In Little House on the Prairie, Charles remarks how foolish it was for the couple they found stranded on the trail to not have a dog, because a dog would have alerted the couple that someone was stealing their horses.