For a serious answer, it creates a form of protectionism for Chinese internet service such as social media, websites, news sites, game services, and so on. Allowing more money to be maintained within the Chinese economy instead of that wealth being transferred to international companies.
This has allowed China to avoid relying on mega conglomerate services and develop their own home grown alternatives to every sort of website and services you could possibly need.
While this wouldn’t be possible for smaller nation like Cuba, or Vietnam to do because they lack the manpower, population, capital, and resources to maintain such a large base of home grown alternatives, China lacks none of those vital items, and thus it makes sense for them to develop their own.
For people that criticize the Great Wall, ask them how the Western internet developed. Due to being the first on the scene, the American internet essentially developed in its own bubble and “firewall” by extension of having virtually no other competitors until the early 2000’s. This allowed conglomerates like Microsoft, Google, Apple, and so on to take off without threat from foreign competitors. Was it wrong for China to design a similar ecosystem to support their fledgling internet based economy?
This is the equivalent of current French colonial states in Africa having French be one of their official languages.