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.
@St0rmFury lucky, I'm the opposite I can't drink coffee unless it's early morning or I won't be able to sleep at night (and I am not an early morning person lol). I love the taste of black coffee though and generally you'll only find one blend of decaf so kinda sucks.