I attended a writing lecture with Canadian writer Douglas Glover at the Center for Fiction a couple years ago, and he talked about the importance of epigram to stories and how often you find them in good writing. I had never noticed this before, but now I find them everywhere. According to Glover, they function as a sort of shot in the arm to the theme of the story--they help raise questions, introduce paradoxes, and heighten irony, and besides that, they are delightful to read and bump up the quality of the writing.
Jenny and the Jaws of Life is full of fantastic examples, but here are a few of my favorites....
From "Melinda Falling": "Mother was a good woman with execrable taste."
Also from "Melinda Falling": "It had often struck me that while we view the pairing of lovers with benign speculation, even envy, there is nothing so baffling, so grotesque, as another man's choice of wife, or a woman's of a husband."
From "My Father, at the Wheel": "This is a very old story, the one about daughters and fathers. It ends in marriage, and the promise of renewal. So it must be a comedy."
When I look at these examples, it also strikes me that they serve to poke fun at the narrator, whose distance, biases, and judging function become all the more apparent in quips like these. Maybe that's part of the epigram's appeal--it's both witty and unwitting, in the sense that it reveals as much about a narrator's own flaws and disappointments as about how the narrator wants to present him or herself.
Quotes: Sorry, no page numbers again, since this if from my Kindle version, but I've included the short story titles for your reference.