What Killshot said. If I shot on level ground most of the time, I wouldn't bother. But most of the time, the ground I'm on is canted 5-10 degrees, the target is canted 5-10 degrees, and the visible horizon is canted 5-10 degrees. All in different directions. What appears/feels level can be off by quite a bit. A bubble level makes sure my scope is level with gravity. The further out you go, the more cant will affect the POI.