Most people say that this is a book that criticizes the conservative businesspeople of the 1920s. The major characters are unhappy with their lives and their jobs.
Perhaps this is why Lewis uses images of work and industry to create the feeling of the first stirrings of the city. He makes us feel these stirrings by talking about the look of the office buildings and of the trains. Then he moves along to the people. All of them (except the rich people in the limo) are workers. One group or type of worker is getting off work and another is starting to work.
So Lewis uses images of work and industry to create the image of the new morning.