Why do people exaggerate in serious contexts? The logically prior question is: What is exaggeration, and how does it differ from joking, lying, bullshitting, and metaphorical uses of language?
Donald Trump in the first of his presidential debates with Hillary Clinton made the astonishing claim that she has been fighting ISIS all her adult life.
Note first that Trump was not joking but making a serious point. But he couched the serious point in a sentence which is plainly false and known by all to be false. So he cannot be taxed with an intention to deceive. Since he had no intention of deceiving his audience, and since the point he was making (not merely trying to make) about Clinton's fecklessness 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.
Our man was exaggerating. That is different from joking, lying, and bullshitting.
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 Trump exaggerated, though he said something literally false, he managed to convey to his audience the true proposition that the Obama-Clinton response to ISIS was and is a failure.
But I wouldn't want to say that the Orange Man 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 our example, the sentence meaning is that of a falsehood. The speaker, however, using a literally false sentence means something different from what the words 'by themselves' mean, and manages to convey a truth 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.
But the main thing in politics and life is that exaggeration erodes credibility. He who exaggerates betrays an inability or unwillingness to adjust his discourse to the world as it is.
Trump could easily win the election if he could get a grip on his rhetoric. But he can't and he won't.
Here I catalog three specimens of exaggeration by well-known philosophers.