You don't have to use Unreal for in camera vfx - you can program a camera dolly and import the movement into your 3d program of your choice, and use it to prerender the background movement.
But that only works if you already know the exact movement and you basically end up playing a film on the background screen walls. You can't change anything of this on set anymore. Using real-time rendering with Unreal or whatever gives you more flexibility in exchange for visual quality.
