The modern safety elevator was created so that both people and building materials could be lifted high up during the construction of taller buildings. this is an example of:

I would say this can be classified as ergonomics whereby it has to do with the complementary relationship between the user, the equipment and the environment. In this case, it is utilizing a fast safe way to transport men to different floors of a building in construction and combining the elevator for lifting materials is maximizing its efficiency ie having a dual use. If the workers had to climb stairs to get to work then they would get too tired and could have an accident and also at the same time they get to their work site or shortly thereafter their materials could get there too.

Because a model would make the investigation easier to understand