Symbolism, Imagery, Allegory
The story Leonora inserted is inserted seemingly randomly in the middle of Joseph Andrews, right after Joseph and Adams split up to travel. The only connection this story has to the book's plot is that the coach passes right by Leonora's house. So what's the deal? Why devote a chapter to Leonora?
We're Not Saying She's a Gold-digger
Basically, Leonora's a gold-digger. She's totally fickle: she throws off her first fiancé, Horatio, just because a new guy appears with a coach and six horses. Okay, we'll say it straight: Leonora seems to be in it for the money. Or maybe she's just in it for the prestige that comes with marrying a fancy Frenchman: "Aye, but Bellarmine is the genteeler and fine man; yes, that must be allowed" (2.4.35). Now, wealth-obsessed women don't feature prominently in Joseph Andrews, so it seems odd to introduce Leonora at this point.
Or does it? Joseph is practically the opposite of Leonora in every way. If Joseph Andrews is the story of a man who preserves his virtue, the story of Leonora is an allegory about what happens when someone is willing to compromise her integrity for the promise of material objects.
Of course, all of that vanishes into thin air once Bellarmine can't get a dowry from Leonora's father—so Leonora does pay a price for her greedy ways.
Don't Forget Horatio
Horatio, the guy who doesn't have the coach and six, is Leonora's spurned lover. Let's not forget his significance to this story, especially since he's the only one who isn't mercilessly satirized. He loves Leonora despite her fickleness, even though he sees the writing is on the wall when Bellarmine appears. And sure, he could have her after Bellarmine ditches her for good, but why bother with an unhappy marriage?
See, Horatio conducts himself on the straight and narrow. "[…] Give yourself no more airs, for you see I am coming for you," he tells Bellarmine as soon as he finds out his involvement with Leonora (2.4.42). That's shades of Inigo Montoya right there.
All this is to say that Horatio is the one shining example of a gentleman in Leonora's story. He doesn't pull punches and he doesn't fall for Leonora's doe-eyed act. And yet he's still a little unhappy at the end of the story: he wouldn't ever go back to the scheming Leonora, but he still sighs when he hears her name. We're thinking you could read this story two ways: either it's no use to fall in love, or if you're going to fall in love, you should make sure to fall in love with a lady like Fanny. Luckily, Joseph's nabbed her already.