Is it unnecessary to use FixedUpdate for a non-physics game?

You should use Update() for things that change every frame, for example to achieve smooth motion you should update your card / camera position there. Even if it's not recommended (because of its linkage to physics), you are free to move more computation-expensive logic (or the one that's more dependent on the rate of calling) into FixedUpdate.

For example, let's say you have a Turret gun that aims and shoots. You may then: Use FixedUpdate to calculate angle to the current target Use Update to smoothly rotate gun to that angle Use even slower rate method (InvokeRepeating or coroutine with loop and wait) to test if target is valid / optimal and choose different target eventually