Why ply? Plying, as many spinning teachers will tell you, covers a multitude of sins—at least of the textile kind! It does this in a number of ways: it makes the finished yarn more consistent, much stronger, and certainly more stable.
The consistency comes from averaging out the diameters of the singles as they are plied together. When we ply, thin and thick places will often merge together, and, when they don’t, there are good plying tricks to help this happen. Of course, the more singles used to make a plied yarn, the more opportunities available to make an even yarn.