If using wool and you spin it with twist, you need a ply to even the twists out and not get something that crinkles in knitting or weaving. If you do a low twist single, you won’t need the ply but it is generally a thicker yarn that is used for soft garments and won’t stand up to wear.
Flax or linen yarn doesn’t need a ply because when it dries after spinning or twisting, the fiber sets the twist and doesn’t curl back on itself to the same extent so it doesn’t need plying in the same sense as the wool does. If you weave with single linen, it holds it under tension enough that the twist is gone. Or perhaps I also need to spin the linen with less twist that I have been in the past.
I’d be curious to see if other plant fibers like cotton also don’t need to be plyed for the same reason.