This new checkbox allows the landscapes lighting samples to be used more accurately for the placed grass. Think of this like the same way it would be for any Movable object being lit by the indirect lighting samples in a statically lit environment. There is no shadow casting happening with this.
Since you’re using a Stationary light you can still have shadow casting by setting the Stationary Dynamic Lighting Distance and the indirect lighting cache will provide the samples needed to the grass while still shadow casting. By default the Stationary Directional light does not have any value set for it’s dynamic shadow distance.