One of the basic rules of traveling light is to have a piece of luggage for just what you need on your trip. But there are instances when you have extra stuff to bring back on your return journey (like maybe some large textiles from the local craft markets, some large gifts for your friends…). Of course, you could take with you a half empty luggage (especially if you have an idea of the size and weight of the extra stuff you intend to travel back with) but it is an unnecessary hassle, and exposes you to potential luggage fees or difficulties to find room for your luggage in the train, bus, plane…