just a warning , anything around 3khz can get irritating if you arent careful. from what i know it's because thats the frequency range that babies cry in, so human ears are more sensitive to it
I think its more about balance. Its the most sensitive region so it's also the frequencies that naturally sound the closest to the human ear. Boosting in this area(well 1-5k) can make sounds be more upfront in the mix. It is also why transient information is most apparent here too. In this case he is making the vocal more clear and audible and have more of an attack. But yes being careful not to overdo it to the point of harshness is important.
