Why can't I apply an Empty's rotation? I’m following a tutorial which has got me using images, imported via the use of Empties.

I start with a new project, deleting the default cube and adding an Empty of type ‘Image’.

I open the Empty’s image, and it appears, lying flat in the XY plane. I apply a rotation of 90 degrees about the X axis. The image is now ‘standing up’ in the XZ plane.

I could very happily leave things as they are, but my inner neat-freak wants to apply this rotation.

With the Empty selected, I press CTRL-A and select ‘Apply Rotation’. I’m expecting the Empty to keep displaying the image in the XZ plane, but for its rotation values in the Transform panel to be zeroed out. Its rotation values are, indeed, now all zero, but the image lies back down in the XY plane.

If I add, say, a Cylinder primitive, it’s orientation doesn’t change when I apply its rotation. 

There’s clearly something about the logic of Empties and the way they behave which I’m not understanding. What am I missing?

Thank You in advance to anyone who can throw some light on this.
Empties only store position, rotation, and scale. You are thinking of them as if they are mesh objects, but they are just the transform component of objects, and thus, you cannot apply rotation and get the result that you are expecting.