The D3100’s electronic rangefinder is mainly a manual-focus aid. When it’s enabled and you are focusing manually, the camera can show whether the subject is in focus and, in many shooting modes, which direction to turn the focus ring to reach focus.

So the main time to turn it on is when you want help focusing a lens manually.

Based on the community answers:

• It is useful with manual focusing.
• In modes like A and S (and possibly P), it can indicate both focus confirmation and which way to turn the ring.
• In M mode, you may only get the focus confirmation dot rather than directional guidance.

If you mostly use autofocus lenses in autofocus mode, you generally don’t need it. If you often switch to manual focus, leave it on for extra guidance.