Fun fact: that hole allows us to drink the coffee without having to suck it out of the cup or crushing the cup with our hands.
Try drinking coffee with the hole covered, and you'll know what I mean.
The underlying physics is related to air pressure.
That's actually super interesting. You mean the little tiny one on the other half of the lid, right? But I think the hole they are talking about in this chapter is the big one you take a sip out of. She probably usually takes the lid off or something.