This is one of the elements of the indirect control of diagram layout: the fact that connections cannot start from the top or the left side stimulates the development of the diagram from left to right and from top to bottom. At first it seems awkward for building diagrams, but actually it is very convenient for quick reading of the diagram and understanding it, as we subconsciously remember the most probable positions of ties and look right and down to find outputs without losing focus on the evaluation of other sides. While constructing, one can always choose a layout that takes this limitation into account, but in the end we get a big bonus to the readability of the diagram on the subconscious level.
