It is important, however, to realize that another reason for the use of "types" in the novel is that this is not a novel about many characters; instead it is the journey of one man. It is a bildungsroman: the personal education and development of a single person (Martin Arrowsmith, in this case). Moreover, other characters are both a vehicle for satire and a way for Lewis to provide further create the character of Martin, either by placing these other characters in juxtaposition or through a connection to him.
Furthermore, as a side-note, Zenith, the largest city in Winnemac, is also the city in which the character of Babbitt from Lewis's earlier novel entitled Babbitt lives. This is a device that Lewis is using out of cleverness and also for the purpose of tying the themes of his novels together.