The traders find it helpful to tow the cedar logs instead of storing them inside the ship to help lessen the load of cargo on the ship. Logs are heavy even for just a log and so storing them inside the ship would one, increase the weight of the ship which will let them sink if they do not manage its carrying capacity of weight, two, it minimizes the area of the ship to be used instead of placing other items that would maximize their transport and three, if anything else goes wrong, at least they have another life support that they can easily get if ever they are drowned. These goes back to the late 1500's where transportation of everything including people and materials only uses ships or boats.