Why are there holes in the drainpipe of the sink?

Answer 1

Sink drains have holes on the sides to mate with the overflow drain above the drain flange. While most water goes down the top, some may overflow and go behind and though a channel in the wall of the sink. Then that water goes through one of the holes on the side of the drain and goes down. When you are installing it, you should try to align the holes with the overflow channel. You might have to compensate for rotation as you tighten the flange. In other words, you might want to partially block it to where any rotation will cause the pipe to rotate into the correct position.

This can be inconvenient when you are attaching the drain kit to a sink that has no overflow holes. In that case, the holes in the pipe serve no purpose, and you have to make sure things are sealed well so it won't leak between the rubber flange and the holes on the side.

There may also be a single, threaded hole further down. That is for the lever for the popup valve for the drain. The lever pushes the drain plug up and/or pulls it down.