Honest question: what is it that Chinese officials fear would happen, if they let their citizens have free access to the internet?
The censorship blocks alternate opinions from forming around issues everyone knows about - for example everyone knows about the Cultral Revolution in China, but the interpretation of it is strictly controlled by the gov in a way that protects the CCPs legitimacy and Maos legacy.
It also stops news about the broken state of the system ever being revealed. For example corruption - occasionally a corrupt official gets 'outed' on social media and when that happens, the government often has to do something - the official gets sacked or investigated. BUT, it's rare it gets to that state. Now imagine if they had a free press or free internet - they'd have to sack half the government within a week, given the scale of the corruption!
Finally, most of the censorship is also aimed at killing activism before it even starts. Online movements never get off the ground, people never coalesce around an issue, the power of the people is never allowed to gather and grow to rival the power of the state/police/political elites. Censorship often aims to keep people (politically) ignorant, isolated and apathetic.
Have a read of this recent article by the BBC to learn the silly (by our standards) lengths they go to guard the myth/backstory of the President ....
http://www.bbc.com/news/magazine-34549744
The journalists were amused by the length they were willing to go to precisely craft the creation myth of Xi. The journalists got the impression that all the visitors/pilgrims they saw there were actually security personnel dressed up.
For that reason, the Chinese cannot have those things. The whole point is that the same people must stay in power. So you can chat online all you want about hobbies or celebrities, but nothing about how the system is broken. And forget assembling in small groups in an effort to change things.
The issue here is feedback loops. In the west, closely-knit political groups can "own" the government for periods of time. Once they screw up -- and they all do -- the voters can have new groups come peacefully into power. There's a way to be wrong. The Chinese have a feedback system where change happens inside the party, but it's much more creaky and brittle. Open net access would expose all of that by getting people talking and comparing political systems.