lianbeng noted shouting is a form of strategy to make opponent stressful so lose focus.
bruce lee did that many times, remember?wataaarrr!!!
That was also what Monica Seles did before Maria Sharapova; grunting and screaming with ear membrane-shattering decibels. Guess it's part habit to steady one's own nerves while at the same time, rattling the opponent and in a way, getting on their nerves!
FTW, no doubt originally from China is now lawfully a Singaporean. Imho, she handled the huge pressures very well and deserve all her rewards and medals. She worked hard for her money, whether one likes it or not. We should give credit where credit is due.