The most important characteristics of a society are economy, culture and education. To understand these, studying its major cities is the best choice.
Firstly, the major cities' economical features are the best example of the economy of a society. For example, the economical characteristic of USA is "market economy" and extensive competition is very important characteristic. It is very convenient to collect examples to show the extensive competition in its major cities such as New York. To start with, the extensive competition is observable in shopping. In NY, if someone wants to buy clothes, thousands of clothing stores are throughout the city, often in a shopping mall, there are dozens of stores that are selling clothes. A buyer will compare many clothes in different stores before choosing the most favorable one. Moreover, the extensive competition is also noticeable in banks. a lot of famous banks, such as Citibank, HSBC, Morgan Stanley, are located in NY. The similar even same financial products created by these banks are so many that a customer has tens or hundreds of choices. To persuade the customer to buy their own products, these banks must compete with each other. In other aspects, such as theaters, hotels, restaurants and so on, the extensive competition is remarkably obvious in NY. Other major cities, such as Chicago and Los Angeles, have the same economical characteristic. So, obviously, to understand the economical features of a society, studying its major cities is the best choice.
Secondly, the most important cultural characteristics of a society can be found in its major cities. For instance, cultural diversity, the most important feature of Chinese society, can be easily found in its major cities: Beijing, Shanghai, etc. In Beijing, Beijing Opera, the Great Wall and the Forbidden City are the best examples of Chinese traditional culture；Beijing 798 Art zone，movies， POP Music， fashion shows belongs to Chinese modern or popular culture. Many western operas, symphonies, movies which can represent foreign culture are being performed every day. In other major cities, such as Shanghai and Suzhou, the traditional culture, popular culture and foreign culture are also presented simultaneously. So, obviously, to understand the cultural features of a society, studying its major cities is the best choice.
Finally, to study the educational features of a society, the best examples are its major cities too, for which the educational system of a major city is a miniature of the educational system of a society. For example, in Beijing, from usual schools such as kindergartens, elementary schools, junior and senior high schools to famous universities, colleges and some training schools which are rarely can be seen in small cities and countries, are all can be found in the city. According to a survey, there are nearly 800 different schools in Beijing. Other major cities such as Shanghai and Suzhou are similar to Beijing. So, obviously, to understand the educational features of a society, studying its major cities is the best choice.
To sum up, although it is not a good choice to study major cities for understanding some aspects of a society such as agriculture, it is the best choice to understand the most important characteristics of a society - competition in economy, cultural diversity and educational system.