When you click on a non planar face using the reference geometry>plane option, you can create a tangent plane. This tangent plane will be placed arbitrarily until a second reference is selected. By using a sketch point, these planes can be easily positioned in the desired orientation.
In the case above, you can see that a sketch point was used on the outside of the cylinder, to position the plane. This can be useful for creating an extruded cut normal to that outside face. This method also works well for more complex geometry, like the boundary feature shown below.
By using a 3D sketch to place a sketch point, a tangent plane can be placed at that point with respect to the curvature of the surface. Check out the video below for a more detailed tutorial: