The narrator of Tom Jones occupies a pretty weird place in the novel, since he doesn't participate directly in the action of the book (like a central character would), but he does come across as an opinionated, three-dimensional character in his own right (unlike the average third-person narrator).
In our section on "Point of View," we discuss the narrator as a fictional stand-in for Henry Fielding, the real-life author of Tom Jones. We also analyze his limited narrative perspective and what it means for the novel. If you're interested in Tom Jones's unusual narrator, we definitely think you should go check out the "Narrator Point of View" section—go ahead, we'll wait.
You're back? Good. So, we don't want to repeat what we've already said about the advantages of having a narrator who seems human, instead of some all-seeing, all-knowing, impersonal teller of the tale.
We want to add just one last thing that we admire about Tom Jones's narrator: he is really self-deprecating and funny. We owe a lot of the novel's sarcastic humor to the particular character of this guy. Let's look at a passage:
As this is one of those deep observations which very few readers can be supposed capable of making themselves, I have thought proper to lend them my assistance; but this is a favour rarely to be expected in the course of my work. Indeed, I shall seldom or never so indulge him, unless in such instances as this, where nothing but the inspiration with which we writers are gifted, can possibly enable any one to make the discovery. (1.5.4)
Immediately before this passage, the narrator outlines some of Bridget's personal motivations in pretending to be reluctant to take care of baby Tom, since she doesn't want her brother to think she's doing him a favor for free. She wants Squire Allworthy to acknowledge that he owes her one.
The narrator tells us that he's jumping in to explain Bridget's motivations because we need to benefit from his special writer superpowers of perception. We, as lowly readers, can't really be expected to understand why these characters act the way they do. In other words, the narrator is busting our chops. He's teasing us for being too dumb to figure out what's going on with Bridget without his help.
But we know, from the narrator's heavily sarcastic tone, that he doesn't really think that, "nothing but the inspiration with which […] writers are gifted" can possibly reveal character motivation.
We also know that he is being tongue-in-cheek when he says that he will rarely interrupt the narrative like this. In fact, the narrator interrupts the plot of Tom Jones all the time to offer his own explanations or commentary. This passage is funny and a little bit self-mocking: the narrator implies that he knows he explains too much. He's inviting us to laugh with him at his own tendency to talk (and talk, and talk).
The narrator wants us to like him, and to show that he likes us, his readers, in return. And it works: we often feel as though the narrator is drawing us into a chatty, witty, hilarious conversation. The narrator's unique characterization allows him to talk with us, instead of dictating at us, as the novel's plot unfolds.