We have put together a list of reasons why these holes exist on the side of a sink drain.
Overflow
The primary purpose of the side holes is to make sure that flooding of the sink doesn’t happen. In case the main drain is unable to keep up with the incoming flow of the water, the holes in the side drain will help make sure that the water doesn’t reach the top of the basin. However, these holes are not a guaranteed way of preventing a flood because they are quite small. Also, if the drainage problem is because of the pipes as opposed to the main drain, there is very little that side holes in the sink can do.
Design and Location
Different sinks have varying designs along with hole types and where they are located. Some might be on the same side of the basin, whereas others might be located under the faucet or at the sink’s opposite side. The difference in the placement of these holes is meant for aesthetic reasons only.
Cleaning
The side holes, just like the main drain, can get dirty and clogged. If your sink is draining slower than usual or emitting a foul odor, then you need to clean your sink-holes. The side holes can be a hard area to reach, but there are a few techniques that you can utilize. Throw a ¼ cup of baking soda into the drain holes and then rinse them properly using hot water.
You can also make use of hot, white vinegar if you are planning to clean the drains aggressively. You can also spray your regular bathroom cleaner into the side holes and, after a few minutes making use of warm water to clear the drain.