Hume knows that philosophy and morality aren't always the easiest subjects to come to grips with. That's why he turns to imaginary scenarios or fables to explain and back up his points.
One stand-out example is the tortoise and the hare, which is one of Aesop's Fables. Even if you haven't read it (it's a quick and easy read that you can check out here), chances are you'll know that it's about a plodding tortoise that manages win a race against a speedy hare. It's about more than that, though. The whole point of fables is to teach a moral lesson and this is one of the most famous examples. In this case, it's meant to demonstrate the perils of being complacent and over-confident.
Hume could've just explained that being focused and determined is better than being rash and losing sight of long-term goals. But, frankly, that would be kind of dull. As soon as he mentions the tortoise and the hare, however, a light bulb goes off in our heads and we instantly get what he's saying.