You can use it to explicitly enable/disable direct light sampling of a mesh light.
Why you might need this is explained in the tooltips:
- auto: "Direct light sampling is disabled if the mesh has more than 256 triangles"
"Enable direct light sampling: Trace a shadow ray for each triangle of this mesh (expensive if mesh has many faces). Use this option if the mesh has more than 256 triangles and is the primary light source in the scene"
"Disable direct light sampling: Improves rendering performance of highpoly light emitters if they contribute only little light to the scene"