Halflings are one of the trickiest races to make unique. They really seem to come in two varieties: the rural, simplistic, large-familied, Tolkien-style Hobbits, and the sneaky, urban, vagabond, every-single-one-is-probably-a-thief D&D style Halfling. Attempt to blend the two... and well, you end up with Kender, I guess - big-familied simplistic rural thieves. Try to play against both types and you either get something unrecognizable or you stray into D&D-era Gnome territory - "We don't know what to do with these things other than make them some other race's poorer cousin".
I actually kind of like the "vagrant race" bit of Halflings that 3.5 and PF tend to suggest, minus Golarion's bit about them being stuck mostly as a slave race, so I went with that. Tying them to Humans - changing their racial subtype - was kind of a knee-jerk decision, but it helps to tie their shared origins together, I think; I may make something more of it as Paziou and its history gets more and more developed. I'll get into the individual geographical and political locales in greater detail later, but for a quick glimpse, Paziou was designed to be Finiens' place for "ancient lost civilizations", along with the crown of Stormwind where Unknown Kadath sits locked away behind the Mountains of Madness. So adventures that take place in any sort of high-magic or lost-art fallen/forgotten/overthrown/etc. culture are free to get dumped into Paziou, where they can be "discovered" by modern explorers and adventurers as plot permits. FR's Netheril? Paziou. Greyhawk's Suel Imperium? Paziou. Golarion's Thassilon and Azlant? Paziou.
So, that out of the way, back to Halflings. So we have an entire race that is, barring some exceptions, immensely nomadic and prone to wandering, highly individualistic but with strong familial bonds that extend beyond direct relations, and have a mechanical racial bonus to getting into and out of places they shouldn't be, getting their hands on things they shouldn't have, and hearing and seeing things they shouldn't observe. What on earth could an entire race have an either inborn or trained-from-youth skill in subterfuge, stealth, and observational skills for, that would go hand-in-hand with their penchant for never staying in one place very long? What are they looking for, trying to find, or wanting to see?
Well, why not Everything?
So that became it - the great purpose behind all Halflings being wanderers and being just naturally good at sneaking around. Their culture, as a race, is bound around the idea of seeing everything there is to see and bringing it back somewhere to be gathered, checked for accuracy, and recorded in one big collective lump sum of information. (More accurately, there's actually probably multiple copies in different parts of the world, the caretakers of which likely meet up in gatherings of two or three teams at pre-arranged times to compare info and check against one another, picking up missed bits and making corrections.) Participation isn't forced, hence how some Halflings are content to settle and raise families and become stable parts of a community, but there's some great honor and respect to be earned from participating in The Story.
For one, you've become an active, involved, and direct participant in History. By submitting something to The Story, you've put something into the project that, perhaps, might have never been there without your involvement. Think of how much has been lost - or, perhaps worse, only reported second-hand - because some Halflings weren't interested in participating in The Story, or because no Halflings were part of the party that went on this grand quest or defeated this great evil or uncovered this ancient treasure! Being a Story-Gatherer makes you indispensable in a way that only a real historian can appreciate.
And secondly, it makes you a recognized representative of your people wherever you go. And for Halflings, that's a big deal. They may wander constantly, traveling alone or in small units, or as a lone observer in groups otherwise composed of non-Halflings, and can observe firsthand how badly other races usually treat strangers, even those of their own species. This is never the case with Halflings. To a Halfling, you're family, even if you've never seen these people before and they never you. And that goes double if you're a Story-Gatherer; even those who don't participate in the Great Work will (usually) recognize one who does, respect them for the sacrifices they've made in the name of the race's great doing, and provide what aid they can afford to spare, even if it's merely encouragement and a warm bed for the night. Halflings have learned not to expect that from other races - heck, the other races can't even manage to regularly offer it to their own kind! - and if anything it's only strengthened their determination to do well by their own kind, especially those who are doing the race's Work.
And a Story-Gatherer who does wrong by their would-be hosts? That's probably the greatest crime a Halfling can commit, depending on the severity of the infraction. And word would get back. Other Story-Gatherers would eventually learn of it. The information would make it back to the collectors, be spread between collection groups, and from there between the Halflings themselves. It wouldn't remain a secret for long... even murdering the hosts to hide the secret wouldn't be a cover forever, because other Story-Gatherers would certainly investigate. And they'd be good at it. After all, their entire racial culture is based around this. They have the mechanical bonuses to reflect it being ingrained in them from as young an age as to make it seem inherent.
Such a betrayal would potentially make the perpetrator of the crime a pariah, one whose word could never be trusted, one whose Story was nothing but Lies. Should the crime be severe enough, or repeated through enough altercations, they might even be excised from the pages: their authorship revoked, their contributions torn from History (or simply uncredited, if collaborated by a more trustworthy source), and their name and their wretched deeds would be in the records for all to see, for all eternity, an unforgettable and unforgivable shame, and a lesson for any who would think to do likewise.
I'm not sure, even from a GM's and Worldbuilder's standpoint, what the ultimate purpose of The Story of Everything is. It's a project that by its very nature can never truly be completed; when it's finished, it'll be because there's nothing left to write, as Finiens reaches whatever end fate has in store for it. I imagine it's not all one book, of course, and completed volumes surely make excellent and highly-reliable historical record, though. Maybe I'll figure it out one day and decide something. Maybe one of my co-worldbuilders will. Maybe someone in the comments can come up with an interesting explanation.
Or maybe all the rampant speculation is the point. =)