In my current 4E sandbox I determined the players needed some way to gauge the strength of potential opponents before they actually engage them. In a sandbox the encounters are not placed with the level of the characters in mind so it is easy to be outmatched quickly if the players are not careful.
I decided on Insight because this is more than a simple Perception; it is more about coming to a conclusion after observation. The character watches an opponent, sees how they carry themselves; how self-confident they are, how they handle their weapons, how alert and on guard they seem.
This use of the skill is used when there is a chance for conflict in a standard encounter format. It is not meant to determine if a village of 100 orcs will be dangerous (they will be). This is more for determining if the party can handle the orc guards at the gate of the village or one of the patrols around the village.
Make an Insight roll to determine how dangerous a group of potential opponents could be.