Understanding Lyra's upper body and lower body split

I’m looking at Lyra’s main animation graph. I wonder why it plays upper body animation by both means of Layered blend per bone, and means of Additive.

Take reloading as example, in montage, it play through both UpperBody slot (for Layered blend per bone) and UpperBodyAdditive slot (for additive).

To prove it, I tried deleting either one of the branch while keep another one, it still works with no apparent difference.

Why not simply use one slot to receive all upper body animations?

Also, other actions, like firing the weapon, don’t use any of these two slot. It uses the slot FullBodyAdditivePreAim at the end.

Why do similar behaviors use difference techniques?

The name is UpperbodyAdditive, and its core purpose is to add detailed leg animations.

Since the UpperbodyAdditive slot is always overridden by the UpperBody slot via the PreBone node, only the lower-body additive animation effect remains functional.

When the character is on the ground, the movement blends in the additive leg detail effects; when airborne, only the upper-body animations play exclusively.

That makes sense. Thanks.