They’re dickies. Mock-turtlenecks are what Steve Jobs wears. (The name “dickies” has since become associated with the clothing company of the same name.)
My Grandfather occasionally wore them, and I owned one as a kid (not sure if I ever wore it.) It is indeed to give the impression of a full sweater, without the heft. (If that seems weird, think of it as the functional equivalent of a scarf.)
The really weird thing is that it really stumps the GF when I ask her about them (she’s an absolute savant about certain things, and fashion is one of them. She blanks on this one, so I’m guessing it wasn’t ever a high-fashion thing.) It seems to be a sixties/early seventies era item.