You can take the money out when you reach retirement age, not before.  But when you pay it, you get access to subsidized health care in state run hospitals.

In regards to you being deducted 20 % of your salary, it sounds high.  Different cities have different rates, and from whatever the rate the employer must pay a larger portion, and you pay the rest. 

For example, I am told (and I am not 100 % sure of the numbers) in Shanghai is 30 %, out of which the employer pays 22 % and the employee pays 8 %.  I do think in Guangzhou is 5 % for the employee.  So based on that, I feel you are been deducted too much.