That's like how the symbol @ used to be called "commercial at" (this is its Unicode name). This is because of how sales were listed on receipts and such, think "4 apples @ 50¢ each".
Using symbols like # before a quantity, @ before a unit price, or ⌀ before a diameter was considered critical for minimizing confusion. I think it was meant to work like a sort of Hungarian notation for numbers, so if someone’s transcribing them into an order form or something, and they find themselves copying a diameter into a price column, they catch themselves on the type mismatch.
It never seemed like there was much room for ambiguity in any of the lists I wrote up, but I guess when you screw up an order to a steel supplier and get the quantity mixed up with the length, that can be a pretty expensive mistake.