This rather odd 747 picture is from a quick test I did to make sure the shadow options in Plane-Maker were working right after beating the shadow code silly with a hammer. The wing objects have been marked "interior only" for shadows, and since we are in an exterior view...the wings don't cast shadows. 🙂
Now this is a totally silly way to use the feature, but there is a legitimate use: mark as many of your interior objects as "inside shadow only" as possible; for example, in the 747 the interior passenger cabin object can be marked as interior only - it doesn't cast meaningful shadows on anything outside the airport.
By marking an object as no-shadow in Plane-Maker you save the sim the work of drawing the object, which is good for fps. If your airplane is used for an AI plane, this makes AI plane drawing less expensive.
In fact, you save it the work of drawing it multiple times. X-Plane using a shadowing technique called "cascading shadow maps". Basically X-Plane renders different parts of the world at different resolutions, so that the closer shadows (that can be seen in more detail on screen) have a higher resolution. The user's plane ends up being drawn in a lot of these rendering passes, and as a result the cost of high-geometry-count objects in an airplane can be amplified several times over by shadows. So savings in object-shadow count matter!
(Given the choice of turning off shadows in Plane-Maker or via GLOBAL_no_shadow, use Plane-Maker; it stops drawing earlier and thus saves more CPU.)