There are two related, but slightly different, reasons for this.
First, the Model T has a place in history because it started the transformation of the United States into a society that depends on the automobile. Since the Model T was so affordable, many people were able to buy them. The United States started to be transformed physically (more roads, many fewer horses, and eventually sprawling suburbs) and socially (people came to see cars as a status symbol, for example). The changes to American society alone make the Model T important.
Second, the Model T and the assembly lines that made it also helped to create a consumer culture in America. The idea of the assembly line allowed goods of all sorts to be made more efficiently and cheaply. As goods became more plentiful, American society changed. It began a change that has led to the society in which we now live where we have more consumer goods than previous generations could have imagined.
The Model T and the assembly line led to the complete transformation of the American physical landscape and America’s society.