Why are two-fold yarns often preferred for knitting , rather than single spun yarn?

Question

Why are two-fold yarns often preferred for knitting , rather than single spun yarn?

Answer 1

Single spun yarns are very thin, so it takes more stitches and rows to produce a finished piece.

Answer 2

Two ply yarns usually make a nicer finished garment.  A single ply yarn can retain a bit of twist from when it was spun, so the knitted item can become slanted or crooked as a result.  A two-ply yarn makes a more "relaxed" yarn, so the knitted garment shapes nicely.