This ensures that the animation's initial value will be visible on the property before the animation begins.
To know: we write the final value of the tween to the model layer when we emit the animation.
Consider if we're fading from 1 to 0 with a delay. We'll write 0 to the model layer and then wait our delay amount. If fill mode is not set to backwards, then during this delay period our view's opacity will be 0. We want the view's opacity to be 1 during this delay period. To do so we need to set the fill mode to backward.