Students can learn subtraction with borrowing by using their fingers. These pages are designed to work them up to using their fingers. Remember to ask them, “ten and what makes thirteen?” Eventually they memorize that 13 minus 7 is 6, but until they have that memorized this is a method they can use to get the answer.
Again certain elements of the worksheet do not show up until after you download them. Little hands help the child transition from seeing them as dots to actually using their own hands.