Comment by kijin

Reusing dishes that were served to another customer is absolutely illegal and carries significant penalties if caught. The problem, of course, is that it's difficult to catch.

Many middle-aged and older customers have a habit of mixing up leftover vegetables after a meal, and they encourage others to do so. The idea is that if everyone does the same, the restaurant can't reuse any dishes.

Meanwhile, honest restaurant owners want to assure customers that the their dishes are new. So they serve food in a way that will make it very obvious if it was reused. For example, kimchi is often served in long (~30cm) slices, so that customers will have to cut them themselves, as if breaking a seal.