It is used to drain the groundwater from the slope during times when the ground tends to become saturated with water due to the melting snow (i.e. spring, or maybe summer depending on the elevation). They are installed in cases where there is a possibility of a slope failure which could cause significant damage or injury.

I'm not going to get into the explanation of why, but saturated soils have lower strength than dry soils, so a slope that is stable when it is dry could fail (sometimes catastrophically, such as a landslide) when the ground becomes saturated.