The light on a lanturn’s back bulb is brighter than the one on its front, and thus, it serves only one purpose. To put it a little more clearly, the front bulb is used for a variety of purposes, including navigation and attracting a mate. However, it’s most famous for being used to lure in prey. When a lanturn’s prey grows close enough, the lanturn uses its back bulb to blind and stun its prey, then uses that moment of distraction to go in for the kill.
In short, hunting.
The Pokédex According to Bill 