Overflow

• One of the main purposes of the side holes is to prevent against a flooding sink. When the water is rushing into the sink and overwhelms the main drain, the holes help drain water that has reached the top of the basin. However, as anyone who has experienced an overflowing sink knows, the holes are not a guaranteed way of preventing a flood. They are usually significantly smaller than the main drain. And if the draining problem lies in the pipes rather than the main drain, the sink holes will do little to stop a sink from overflowing.
  
  

Assist with Draining

• Most people don't think about the side holes until they are faced with an overflowing sink. But the holes assist your sink every time you use it, whether you notice them or not. The holes provide an extra opening in the sink to help water drain faster. Consider a can of juice. If you puncture just one side of the can lid, the juice will flow but at a slow pace. However, if you puncture another hole on the opposite end of the can lid, the juice flows much faster. This occurs because you're adding an extra hole from which air can escape. Similarly, that's what the side holes in the sink do to help drain the water faster from the basin.
  
  

Design and Location

• Different sinks have different hole types and locations. Some may be on the side of the basin while others are situated under the faucet or at the opposite side of the sink, closer to where users stand. The designs also vary. Some are a series of holes; others are small rectangles or slits. The differences are merely aesthetic in nature.
  
  

Cleaning

• The side holes, like the main drain, can get clogged and dirty, especially if called into use regularly to handle an overflowing sink. If your sink is draining slower than normal or emitting a foul odor, try cleaning the sink holes. Although it's a hard area to reach, you can take several approaches. Spoon 1/4-cup baking soda into the drain holes and rinse them thoroughly with hot water. Instead of water, rinse the holes with hot, white vinegar for extra cleaning power. Or spray your normal bathroom cleaner into the holes, let it sit for several minutes then rinse with warm water. If you can't direct water into the holes, use a turkey baster to force water down there. In cases of suspected clogs, try sticking the bent end of a wire hanger down the drain to pull out or dislodge the clog.