One of Dennett’s most important claims is that most of what we and our fellow organisms do to stay alive, cope with the world and one another, and reproduce is not understood by us or them. It is competence without comprehension. This is obviously true of organisms like bacteria and trees that have no comprehension at all, but it is equally true of creatures like us who comprehend a good deal. Most of what we do, and what our bodies do—digest a meal, move certain muscles to grasp a doorknob, or convert the impact of sound waves on our eardrums into meaningful sentences—is done for reasons that are not our reasons. Rather, they are what Dennett calls free-floating reasons, grounded in the pressures of natural selection that caused these behaviors and processes to become part of our repertoire. There are reasons why these patterns have emerged and survived, but we don’t know those reasons, and we don’t have to know them to display the competencies to allow us to function.
Nor do we have to understand the mechanisms that underlie those competencies. In an illuminating metaphor, Dennett asserts that the manifest image that depicts the world in which we live our everyday lives is composed of a set of user-illusions,
like the ingenious user-illusion of click-and-drag icons, little tan folders into which files may be dropped, and the rest of the ever more familiar items on your computer’s desktop. What is actually going on behind the desktop is mind-numbingly complicated, but users don’t need to know about it, so intelligent interface designers have simplified the affordances, making them particularly salient for human eyes, and adding sound effects to help direct attention. Nothing compact and salient inside the computer corresponds to that little tan file-folder on the desktop screen.
He says that the manifest image of each species is “a user-illusion brilliantly designed by evolution to fit the needs of its users.” In spite of the word “illusion” he doesn’t wish simply to deny the reality of the things that compose the manifest image; the things we see and hear and interact with are “not mere fictions but different versions of what actually exists: real patterns.” The underlying reality, however, what exists in itself and not just for us or for other creatures, is accurately represented only by the scientific image—ultimately in the language of physics, chemistry, molecular biology, and neurophysiology. Thomas Nagel "Is Consciousness an Illusion?" reviewing Daniel C. Dennet From Bacteria to Bach and Back: The Evolution of Minds, in the The New York Review of Books, March 9, 2017 Issue
Nagel's generous review introduces two key conceptual innovations in Dennett's thought early in the review, first that there is "competence without comprehension" and, second, that there are "free-floating reasons, grounded in the pressures of natural selection that" can cause "behaviors and processes to become part of our repertoire." But while Nagel nicely draws out the significance of this for Dennett's familiar view on consciousness (although he misses that for Dennett the the scientific image is also of real patterns (see 222)), Nagel misses the wider implications of these two innovations: that they pave the way for a version of functional explanation in social science.
Before I get to that, first a terminological point: in his review, Nagel attributes to Dennett commitment to free-floating reasons (see above). But in Dennett's terminology these are free-floating rationales. Nagel is not subsantively wrong, however, free-floating rationales are a species of reasons, which are "tracked" by "evolution" (50). In context Dennett is describing "the accumulation of function by a process that blindly tracks reasons, creating things that have purposes but don’t need to know them." (49) So, free-floating rationales are reasons that do not require being represented for their existence: in particular, they are functions that are a consequence of natural selection. (One way to understand a free-floating rationale is that it is the answer to the why question of reverse engineering. (92))
Okay, with that in place, we can turn to the significance of Dennett's move, once we recognize (or recall) that for Dennett free-floating rationales are not merely a biological phenomenon; he also treats cultural evolution as the cause of particular types of free-floating rationales: "memes, informational viruses that govern infectious habits," (173, emphasis in origina.) Now, in so doing Dennett is especially interested in explaining consciousness and the origins and the nature of language,* but he also wants to show that "Free-floating rationales have been as ubiquitous in cultural evolution as in genetic evolution," (231). This has obvious implications for social science.
In particular, Dennett's free-floating rationales are also a means to revive respectability of discussion of functions in social science (211-212). Social functions simply are flee-floating rationales that are caused by cultural evolution. (Obviously, they may also be represented by agents in society.) That is to say, on Dennett's approach to cultural or memetic evolution provides the mechanism that secures the existence of free-floating rationales. Of course, showing that an attributed social function is the consequence of such cultural selection is not always easy, but that's for another time.
* The passage that Nagel quotes about "the ingenious user-illusion of click-and-drag icons" is, in direct context, about why "Phonemes are not just a brilliant way of organizing auditory stimuli for reliable transmission," but also a benign user-illusion (202).