Convert to Line effect
If you try to apply 'Convert to Rectangle' effect, Ai will limit you minimal width/height to 0.01mm — not zero. It means we can't use it to create 'lines'. And this is something appearance and styles will hugely benefit from.
Of course, it should allow to set a reference point too:
And although we have a Transform effect to deal with the angle, having it for the line is obviously needed.