Thank you, DY for the detailed answer!
“Anyone here know the purpose of a blech? Is it really necessary? What happens if you don’t have one? Oomis, what is the reason for covering the knobs? What does kli sh’lishi have to do with a blech?”
CB, I always thought the blech was to be a lessening of the heat between the burner and the pot (and to make the pot into a klish sheini, which DY pointed out to me, I have misunderstood). The covering of the knobs, which not every blech does, is specifically to make people aware that it is there and prevent people from accidentally turning the fire up or down. Not a bad idea. I put covers on my light switches for the same reason.