Why do people exaggerate in serious contexts? The logically prior question is: What is exaggeration, and how does it differ from lying, bullshitting, and metaphorical uses of language? A physician in a radio broadcast the other morning said, "You can't be too thin, too rich, or have too low a cholesterol level."
Note first that the medico was not joking but making a serious point. But he couched this serious point in a sentence which is plainly false. Since he had no intention of deceiving his audience, and since the point he was making (not merely trying to make) about cholesterol is true, he was not lying. He was not bullshitting either since he was not trying to misrepresent himself as knowing something he does not know or more than he knows.
Exaggeration bears some resemblance to metaphor. If I say, 'Sally is a block of ice,' I speak metaphorically or figuratively. What I say is literally false. But by saying it, I manage to convey to the listener some such proposition as that Sally is unemotional and (perhaps) sexually unresponsive. And when the sawbones exaggerated, though he said something literally false, he managed to convey to his audience the true proposition that total cholesterol levels for most of us need reducing.
But I wouldn't want to say that the good doctor was speaking metaphorically. I am merely pointing to a similarity between metaphor and exaggeration. The similarity may consist in the coming apart of sentence meaning and speaker's meaning. In both examples, the sentence meaning is that of a falsehood. The speaker, however, using those literally false sentences means something different from what the words 'by themselves' mean, and manages to convey truths to his hearers.
So I suggest that to understand exaggeration we need to understand metaphor so that we can delimit the former from the latter. But what exactly is metaphor? That's a tough one. Here I catalog three specimens of exaggeration by well-known philosophers.