For anybody who has used a Vt52, H19, or similar terminal, there is no mystery. Before we had bitmap graphics, we had fill-in forms with ASCII graphics. A vertical line made with solid bars looked uneven, but the gap in the bar was the same height as the gap between a bar and the one above or below it, so it made a pleasing vertical dashed line to match the horizontal lines of actual hyphens and equal signs.

Nowadays, of course, if you want a line, you draw a line, so the gap no longer has a purpose.