These type of 'rulers' are actually termed Machinist's Straight Edges. They are often used in machining metal. The way you use them in quilting would be to measure in with your ruler from say the right. Place your straight edge along the left side of the clear ruler then press and cut.
The reason they are used is because of the need for extreme accuracy when working in metal. Cloth because of it's properties when woven doesn't require this type of precision so it's really unnecessary for quilting. I have some but don't use them for quilting.