1) way of how they travel
before, it is usually horse-drawn carriages, walking, etc.
2) what the general public spend on
as the automobile became cheaper (thanks to the assembly line) more people were able to afford the automobile, and soon the amount of people owning a automobile increased drastically
3) circulation of money
More people bought the automobile, which meant that more money was circulating within the economy, which generally gives a boost to the economy.
4) What is 'necessary'
After a couple of years, an automobile was deemed necessary to have to live the 'American Dream'. Many people bought automobiles to live this dream, which led to the rise of the automobile industries
hope this helps
