It means you can't scan down the left hand side of the editor looking at indentation, you've got to double check with the existence/non-existence of line number

It means you can't scan down the left hand side of the editor looking at indentation, you've got to double check with the existence/non-existence of line numbers. For the same reason that it is easier to read left-aligned text than it is to read right-aligned text, though not quite as bad since not all lines are "wrong".