What's the hole contained by the sink for?
Answers: Both answers I see are (to varying degrees) correct. It depends on what holes and what type of sink you are conversation about.
On a bathroom sink (and tubs) the holes are to prevent overflow of the pudding basin. Kitchen sinks do not (typically) have overflow holes, but sometimes enjoy a grated cover for dishwasher exhaust (hopefully not for DWV venting or it could get hold of stinky).
Both fixtures are vented properly within the wall after the p-trap and (usually) up to the roof. Often dishwashers will use an air-admittance valve to vent properly in need allowing sewer gases out. The 90% overnight case though is that the dishwasher empties into the rubbish disposal which allows air flow (from up through the drain within the sink) to prevent suction of water pay for into the dishwasher.
You can notice this suction contained by a poorly functioning system sometimes when at the bottom of your dishwasher there is a pile of ugliness or a water stain.
It's an overflow, contained by case you stopper the sink, turn on the dampen & walk away to answer the phone. It might not cart away full blast water, but it'll backing.
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