You don't have to tip. So you do get a certain minimum service. In Beijing, no tips required or expected. But, unless you're at a Western-style place, or HaiDiLao, no service expected, either. Should service be required (ordering more items, correcting something, requesting something essential that was forgotten, paying the bill, etc.), yelling "fuwuyuan!" ("waiter"!) loudly will often, but not necessarily, procure you service. So that tip does buy you something.

20% extra for standard service, though, is ridiculous, and I hate it. Nor do I do 20%--even fancy places only do 18% gratuity, at least, the last time I was at one.