When you create a scene, it is in 2D mode by default. This means layers are locked at the same position on the z-axis and can only be rotated on the z-axis. This is to avoid incumbering your workflow with 3D functionalities that are not needed.
You can easily convert a 2D scene to 3D, and back and forth. In a 3D scene, you can move layers on the z-axis and rotate them in any direction. You can also rotate the camera in any direction and make it move across the depth of the stage. Likewise, you can rotate the Stage view in any angle and navigate the stage in its depth, allowing you to look at it from various points of view.
NOTE You can convert a scene to 3D, but not a single panel. When switching to 3D mode, the mode is only applied to the current scene, not the entire project.