The small hole on the sink, which seems unnecessary actually has a surprising function

In fact, this small, modest hole, located about one-third of the way up the basin, is a clever feature that demonstrates the thoughtful and foresighted design of the sink’s creator, anticipating possible scenarios

This small hole is connected to the drainage pipe below. During use, there are many instances when users forget to turn off the faucet (for example, leaving to answer a phone call while waiting for the sink to fill). In such cases, once the water level reaches two-thirds of the sink, it will drain through the hole into the pipe, preventing water from overflowing onto the floor and flooding the bathroom.