What the "ladder" "/ladder" tags do is to make the browser to display all of the text between the tags in a fixed-width font.
Without the tags, letters take up only as much horizontal space as they need, and multiple consecutive 'space' characters get ignored. This makes it difficult to align text with contacts or coils, and vertical lines appear to be placed randomly.
With the tags, each character takes up the same horizontal space whether the character is a 'w', an 'i', or a 'space', and all 'space' characters get displayed.