Whats the point with animation layers?

WHy exactly do we put animation.layer? Are we setting the animation into a layer? But why? What good does this do for us?

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    void Awake() {
        animation["Walk"].layer = 1;
        animation["Run"].layer = 1;

As stated in the docs, animations in higher layers will have their weights distributed first, before lower layers. For example, let's say you have a walk cycle animation that affects the whole body, and a waving animation that only affects the upper body. If they were on the same layer and then blended, then the arms would partially walk / partially wave. If the wave animation was on a higher layer and then the two blended, then the arm would wave normally while the lower body could keep walking.