Ruler allows players to easily be able to identify a "cell" within a level (something like chess notation used by chess players). So when sending email, posting message somewhere or sharing in any other way information about some particular level it is often needed to identify cells.
CTRL+R toggles showing the numeric/alphabet ruler (chess like notation). ALT+R toggles showing the numeric/numeric ruler.
The ruler (especially the alpha-numeric ruler) is very useful in describing a particular cell in the level. Also, if you move the mouse to point to a given cell, it highlights the number and the cell, so you know exactly what the location is.
Example: CTRL+R to toggle the alpha-numeric ruler