To avoid eating the spinning oil from the yarn.
Yarn mills add spinning oils to their wool to keep it from snagging on the machines. You wash it out once you’ve knitted with it. It’s usually just a normal vegetable oil or mineral oil.
But I’m so sensitive that these kind of short chain fatty acids will keep me awake at night. Yes, I’m weird like that.
(I also sneeze 12 times when there’s a draft around my ankles and I hear like a bat.)