what’s the concrete advantage of the average person reading at a high level is

Sixth grade reading is being able to extract information from written word. Below that grade level, you’re basically reading words but none of them need to have logical impact on some greater theme or topic. Sixth grade reading level is the ability to read things that matter. Eighth grade reading is reading at a level where you can apply introspection to the underlying theme or topic that’s being extracted from the written word.

So, and this is simply my opinion, I believe it is important to be able to read things and understand how they apply to one’s self in a logical manner. The ability to extract the impact that the particular piece has is critical to subsequently applying that introspective quality to the piece. So, yes, I believe being able to read at sixth grade level is incredibly important. It is difficult to understand how something applies to you if you cannot correctly extract what the point that is being discussed, actually is, in the first place.

they clearly haven’t needed the skill in their lives

Well a lot of everyday life is not present in those terms. Your employer does not sit there and go “this machine is big enough to stick a arm into it and applies enough pressure to remove that arm from your body” without also following it up with “so do not stick your arm into this machine.” In fact, legal requirements likely dictate a “DO NOT STICK ARM INTO MACHINE” or something similar sign right beside the machine.

There is a massive difference between the utility as a function of labor and employment and utility as a function of operating within a society. So do try to apply this at say a social level. Employers can change the condition of the environment one works in to accommodate a lot of leeway. So do try to think of it less in “what does this higher reading level provide in objective utility in a work environment?”

i also don’t particularly care how well the average adult has their multiplication tables memorised

Well that is interesting that you bring that up because rote memorization of multiplication is third grade level stuff. Things like “all things multiplied by two are even” is higher grade thinking. There is actually a point where you stop thinking of multiplication as some table to be memorized and start seeing it as a pattern that has deeper meaning.

If I add a zero to the end of a number, “3 to 30 or 45 to 450” I have multiplied that by 10, and that has a deeper meaning all over in various engineering domain. In computer terms we call that bit shifting and there’s optimizations in rendering pipelines and memory access that comes from this deeper understanding of multiplication. Or things like (x,y) coordinates, it is easy to just think of it as plotting point on a grid, but at some point you obtain a deeper meaning and start seeing (x,y) as (r,θ) and you begin to have an understanding of vectors and that understanding is critical to literally everything that might have to traverse your GPU. Want to change the heading of an airplane? You can do vector addition to know exactly what will happen when you change that heading.

And even then you stop seeing exactness of math and start seeing the general patterns of it. You begin to understand this kind of change in this variable has this kind of effect. And you can pick that kind of feeling up with on the job training no doubt. But with a deeper understanding of math you begin to understand more than what on the job training can give you, because you can break those actions down into more mathematical terms that can be manipulated easily within your mind, rather than the good old trail and error method (which obviously wouldn’t be a good method for an airplane that you are currently flying).

There is also more to it than the surface level implications of education. Yes, we can just look at the surface level stuff and conclude that the “real” world reigns supreme. But having that deeper understanding, be it in written word or mathematical eloquence, gives us a richer understanding of the world we live in. Gives us more access to the potential of this world. We do not per se “need it”, much like we don’t pre se need things like medicine, refrigerators, guns, bulldozers, and what not. Our ancestors did without them for countless years. But having that deeper meaning gives us access to things that we would otherwise not have. This nice world we have of comfort is not a product of it being forced upon us, it is a side result of various people who went further than the surface level understanding of this world that was routinely offered.

So if you are curious of the advantage, look around you. That is the advantage.