If you don't have a clear ruler, here is why you might consider getting one at some point:
1. It's very helpful when lining things up. Combine it with a grid surface, like my beloved Dritz Superboard, or another surface with a grid, and it is very helpful for getting things straight and making accurate marks.
Here I wanted to mark 0.75" from the top of this piece, so I lined up the fabric with one of the gridlines on my cutting board, and then matched the 0.25" mark on the clear ruler to the bottom of the 1" blue gridline. Now the top of the ruler is perfectly showing where 0.75" is located.
2. You can use it as compass to make circles and arcs. There are guide holes every 0.5" down the middle and it's great for drawing perfect circles!
In this picture I have a pin in the center to hold it down, but you really want to use another pen or an awl (I'll talk about this handy sewing tool another day) in the center pivot hole. The pin doesn't fill in the hole completely so the ruler isn't firmly in held in place, and it can get wobbly as you draw the circle.
I haven't had this very long, so these two benefits are only what I've noticed so far. Maybe other great uses will turn up as I get more familiar with it. If you have one, please leave a comment letting us know what you like about yours.
It's very possible the ruler, cutting board, and awl are available at your local fabric store. If not, they are available several places online. The links in this post will take you directly to where they can be ordered to on Amazon (disclosure: I'm an Amazon affiliate).
Anyway, I'm very happy with the clear ruler and wanted to share my experience with this tool.