I get it!!
When you make HST's by drawing a center diagonal line then sewing on both sides of the line - cut the center, then square up the HST. This ruler will let you mark the two SEWING lines (instead of marking the center line) - and the bonus is that you can pre-cut your squares to the correct size FIRST which means if you sewed them correctly you won't have to square them up later.
So if you use the above method of marking HST's - then this ruler might be a useful tool.
I'm not a big fan of using special rulers for special functions ... I have a 4" X 24", a 6.5" square and a 12.5" square and I do what I need with those. But I LIKE this ruler.