The Great Firewall exists to control the web content that comes into China. It is a tool for censorship and social control.
In recent decades, China has had many admirable achievements. But China remains a repressive society. The Great Firewall blocks a range of Western news content, including the New York Times (from what I’ve read).
The Great Firewall also blocks Chinese sites outside of mainland China that criticize the Chinese government or discuss elements of Chinese history like the famine during The Great Leap Forward (1958–1962).
Computer security researchers have also reported that The Great Firewall has been used to attack dissident sites outside of China.
A side effect of The Great Firewall is that it protects local companies from foreign competition. But this is not it’s purpose.