Tipping puts the burden of service quality feedback on the customer rather than the business manager. It also encourages the server to increase the customer's bill total. Most of the benefits of the practice are exclusive to the business, with no benefit to customer or employee.
Restaurants could pay servers a commission or bonus based on the bill total or for specific items on the customer's bill. Customers could be asked to rate the service they received on a scale from 0 to 10 instead of calculating a percentage of their bill total. But tradition is a tough thing to fight.
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.
Restaurants could pay servers a commission or bonus based on the bill total or for specific items on the customer's bill. Customers could be asked to rate the service they received on a scale from 0 to 10 instead of calculating a percentage of their bill total. But tradition is a tough thing to fight.