Considering ChatGPT was made to provide likely responses based on the training data, it makes sense to include social and contextual cues in the prompts. I associate "quietly think" with instructions from an authority figure. So ChatGPT is then more likely to respond like a person following those instructions.

It works for user prompts, too. When I want it to choose something from a list and not write any fluff, I create a prompt that looks like a written exam.