Think of applying the rotation like you are re-defining or re-setting what you already did, like you are starting all over with no history. So if you rotate a plane 45 degrees, the object will remember it’s rotation and it its mind it has a 45 degree rotation. You can see that in the N panel in the transforsm, the rotation will mark 45 degrees. But when you apply the rotation, it’s like the plane is thinking oh never mind, I have always been this way and he no longer remembers that 45 degree and goes back to no history, in other words, the rotation goes back to zero, and you can see that in the value in the N panel.
