I am not a very good spinner, in fact I am barely above a day one beginner. But over the years of practice, I can confirm that practice makes...better. I can usually keep my strand in one piece. For me, understanding the principles of spinning has driven me to spin finer singles. If I can keep a fine strand going, then getting thicker singles mean just putting more fiber in the drafting triangle. In fact, if I go for too long between spinning sessions, I find myself starting fat for a few inches before I can work down to the fine again.
Spinning a strand of singles means twisting the individual fibers in order that the fibers lock together, at the same time you constantly overlap layers of individual fibers to extend the strand, continuously twisting so that the fibers stick together. Lather. Rinse. Repeat. Sort of like laying a brick wall, how the bricks overlap each other to keep the wall strong. Instead of cement (or mortar), use twist. The more twist, the stronger the individual single (single strand) will be. But it is possible to put too much twist into the fiber. There is a balance of adding just enough extra twist into the single so that after you have made a long enough strand, you can ply that single with another single (or two or three or more) or even with itself, and overtwisting the single so that it wants to curl and knot up on itself when you let go of it.
In our society, most yarn used for knitting, or weaving, or whatever, is plied. In the same way that more twist makes a single stronger, combining two or more singles makes that combined (plied) yarn stronger. Which is probably the main reason society wants plied yarns.
Plies don't have to be made of singles of the same kind of fiber. You can make a plied yarn from a single of wool with a single of silk. Or a ply from a single of one color and a single of a second color. Plied yarn, in addition to being stronger also allows for more possibilities from a design perspective.
But I think singles get a bad rep. Historically, singles have been used to weave fabric on warp-weighted looms, and kept the human race from freezing in the days before central heating. I don't know for sure without doing some research, but singles may have also been woven into the sails that sailed Eric to the coast of North America on his Viking questing. You can spin singles with just enough twist to hold them together, making a soft supple fabric when knitted or woven (but more fragile). You can spin a singles with a lot of twist for strength and long wear, fighting its natural instinct to double back on itself, and weave or knit it into a fabric that wants to bias in turn, but it is as long wearing as you can want. You can spin singles thick or thin, or even"frog hair'. Singles are done when you are finished spinning, ready to use in a final product. Singles highlight the qualities of the fiber used to spin the single.
The size of plied yarn is partly determined by the thickness of the singles used to ply the yarn, and the number of plies to make the final yarn. Plied yarn can be plied in interesting and creative ways. You can ply all the singles together in one go, you can ply one single at a time, twisting the opposite direction each time you add a ply. You can make a plied yarn cabled, or chained. It requires "frog hair" singles if you want a very fine plied yarn, but it is (and was) done. Historically. Pre-industrial revolution. By folks sitting in their dark cottages lit by candlelight. Well, maybe whale oil. And when I say "folks" you should read "women and children". The only time I can think of when european men handled singles and plies, pre-industrial revolution (because they took over nearly all the fiber activities when all those machines showed up), is rope-making. But they got to beat the rope with those clubs, so there's that. I do know of some males who engage in fiber activity, and even some males who make a living teaching females how to be fibery (go figure). It's not just the flower child/hippies of the 60s.
Oddly enough, I haven't heard anything about survivalists educating themselves to create cloth with fiber after society collapses; I guess they are going to be wearing furs and leathers.
