So why do you think self-rising flour was created, rather than just selling the ingredients separately? For example, "self-sweetening" cake flour has never existed, with the sugar already mixed in, though that would save as much or as little time.
Was it mostly a marketing ploy, treating it as a special flour with unique properties, rather than just flour with the same old artificial leavening pre-mixed? I know that Horsford in particular was a fairly aggressive marketer.
It does seem from Shawnra's examples that it was primarily popularized as essentially a precursor to boxed mixes, perfect for all those clueless bachelors forced to fend for themselves in mining camps and prairies. Note the quotations about how it could be cooked up into food simply by adding water (not a way we use it today!). Unlike, say, a pre-sweetened flour, this was the simplest material that could possibly be made into a fritter or flapjack.
I suspect its modern popularity is very regional; I never saw much of it or heard about it in California, but here in North Carolina it has a prominent place on the shelves and appears in many recipes. I would never buy it because I prefer an alterable ratio of leavening to flour, but I can imagine that if one grew up with self-raising flour it might seem easiest.
Ah, I just found this on cooksinfo.com: "Self-Rising Flour is mostly used in the UK (where it is spelled "Self-Raising"), Australia and in the Southern US; it is relatively rare in Canada and the Northern States." That definitely smacks of past successful marketing efforts to me.