What's the meaning of phrase "for fun and profit"?

As Hot Licks points out in a comment above, the phrase "for fun and profit" generally comes up in the context of articles or books devoted to transforming an interest or hobby into a source of revenue. A review of Google Books matches from the first half of the twentieth century finds the earliest such match in an article titled "Poultry for Fun and Profit," in American Poultry Advocate (May 1913). Photo Technique magazine (1939) mentions an author (and "experienced hobbyist") who has published books called "Working with Tools for Fun and Profit" and "Collecting Stamps for Fun and Profit"; another title from the same year is Making Your Own Movies: For Fun and Profit.

By the end of the 1940s books invoking the "for fun and profit" catch-phrase had become a genre, with titles including Training for Fun and Profit—Maybe! (1942), Fortune Telling for Fun and Profit (1942), Hobbies for Fun and Profit (1943) [title not shown in snippet view], Dolls to Make for Fun and Profit (1944), Cartooning for Fun and Profit (1945),Creating Jewelry for Fun and Profit (1947), "Metal Craft for Fun and Profit" (1948), "Netting for Fun and Profit" (1948), and Raise Crickets for Fun and Profit (1949).

In recent years, instances of the "for fun and profit" formula continues to be used in the same way—witness, from the five-year period from 2004 to 2008, Collecting Cigarette Lighters for Fun and Profit (2004), Detailing for Fun and Profit (2004), Dream for Fun and Profit (a memoir) (2004), Successful Cocktail Waitressing: A How-To Guide for Fun and Profit (2005), How to Start Your Own Cult for Fun and Profit (2005), Facepainting for Fun and Profit (2006), Cab Driving s a Second Career for Fun and Profit (2006), Changing Faces for Fun and Profit$ (a novel) (2006), Estate Sale Prospecting for Fun and Profit (2006), Casino Gambling for Fun and Profit (2006), Outfox the Kids for Fun and Profit (2007), How to Make Boxes for Fun and Profit (2007), Sweet Potato Queen's Guide to Raising Children for Fun and Profit (2008), and Sharing Secrets for Fun and Profit (2008).

Clearly some of the more-recent titles—like Billionaires for Bush: How to Rule the World for Fun and Profit (2004)—are pushing the irony button rather hard, but the ubiquity of "for fun and profit" as a set phrase in the self-help universe is unmistakable. And an Ngram chart for the phrase across the years 1900–2005 shows that it is still going strong (or was, as of 2005):

The crucial idea here is that you can do something that's fun—and make money from it. So if you already like chickens, or cartooning, or making boxes, why not turn that enjoyment into a side business that can earn you dollars (or at least pennies) a day? That's "fun and profit" in a nutshell.