What are the square cutouts in a bathroom sink drain for?

It obvious I'm a non-plumber to ask a question like this

I just replaced my bathroom faucet and am in the process of also replacing the pop-up drain. I noticed that the drain tube that connects to the pipe below has several square-shaped cut-outs just above the threads that accept the nut that secures the drain pipe to the sink.

Why isn't it just a solid tube?

Also, I am still getting occasional water seeping out of the base of the new faucet. I used the gasket that came with the faucet but no silicone or putty. Does this mean there is a cylinder problem in the handles of the new faucet?

Thanks

They are where the water from the sink overflow goes. Were'd you get the faucet from. As in who makes it?

Thanks for the response. It is an Aquasource model F5120000NP. It was on sale at Lowes. A new faucet shouldn't have defective cylinders.

No it shouldn't but most of that stuff comes from China so.................

Here is a diagram showing the square holes in the drain for the overflow water to get into the drain, that Tom mentioned.
http://drainoutcleaning.com/bathroom-sink-drain-plumbing/

If it were a "solid tube", the sink's overflow would not work, and you would have the slow drainage that "vessel sink" owners complain about. There are many places that the water could come from, but a loose aerator is the most likely one on a new faucet.