Here are the two different threads, same settings, and the same fabric.
The bulky nylon is much soft and fills in the stitch much them serger cone thread. And some stretch to it.
When to us, when you want to use your coverstich machine to give a reverse cover stitch.
And on anything that is worn close to the skin where the stitches will contact the skin. The bulky Nylon does not scratch or itch like thread can sometimes do.
Also helps form having thread pops or breakage, from the bit of stretch it has.