When you launch the extension, you have a few options to add a "ruul":
- Resizable - Allows you to create a box to measure a specific area in inches, centimeters, or in pixels.
- Add a ruler - Select one of the predesigned rulers to put on a web page. The measurements of the ruler are in pixels.
I have used this in finding the measurements for headers on Google Sites, or cover photos in G+. I can see the use of this in yearbook and newspaper classes when measuring out sample template ideas, in geometry classes who are blended and working a lot online through Google Docs and Drawings.
Here's a little .gif to see the functionality: