It's the signal that they have no idea what they're talking about. Always remember everything doctors said people should do to cure their stomach ulcers: the "why not?" list.
"Your medical team has carefully gone over the results of your tests, and we've finally come to the careful consensus that you should keep active, drink more water, get enough sleep but not too much, and eat your vegetables."
"But..."
"Why not? Do you think vegetables are bad for you?"
"Your medical team has carefully gone over the results of your tests, and we've finally come to the careful consensus that you should keep active, drink more water, get enough sleep but not too much, and eat your vegetables."
"But..."
"Why not? Do you think vegetables are bad for you?"
"But doctor, I have a broken leg!"