There are several instances of digressions in Tom Jones.
One example of them is the introduction of each chapter. Fielding gives us a hint of what the reader is to expect in the chapter that he or she is about to read.
Another- In the first 2 chapters of the story, we learn about the background of Mr. AllWorthy, the loss of his family, how he came about his wealth, his position in society, and much more about his sister et al. This, while not directly a part of Tom Jones's own story, is a sermon that Fielding explains is necessary for the reader to know before hand.
Another example (a very good one) was the sermon that Dielding gave Jenni Jones, Tom's mother, on how her decision to get involved in a relationship, have a child out of wedlock, and then abandoning the child, broke with every canon of morality and social behavior, and that she should be shun from society as a misfit and as an immoral person.
This is a good example because the preaching that Allworthy gives to Jones is almost a complete story of its own. It has a beginning, middle, and end, hints to causes and effects, cites on a diversity of matters, and could be read independently from the rest of the story, still making complete sense to the reader.
These forms of long, often a bit wearisome, and highly moral moments in storytelling are often separated from the main topic of the story, but are meant to make a point related to it. In this case, it was the sad way in which Tom Jones was found, and the origins of his being with the Allworthys.