There is a "set rangefinder on" kind of option in D3100.
Are there any special cases when I "should" set the rangefinder on?
I don't have a D3100, but everything I can find says that the electronic rangefinder is a manual focus aid. When the camera is in manual focus mode and you turn on the rangefinder, the exposure meter will indicate which way to turn the lens to focus. So you could turn it on when you want some help focusing manually.
I am speaking out of my experience with a D60. Range Finder is helpful only in case of manual focusing. For modes A and S (and possible P), the rangefinder will indicate not just if the subject is in focus or not, but also which way you need to turn the focus ring. In M mode however, you have to solely rely on the focus dot.