What does Unity.Animation.AddRef() do?

Unity Engine
1 
namespace Unity.Animation
{
    public abstract class GraphSystemBase<TGraphTag> : JobComponentSystem, IGraphSystem<TGraphTag> where TGraphTag : struct, IGraphTag
    {
        protected GraphSystemBase();

        public NodeSet Set { get; }
        public int RefCount { get; }
        public TGraphTag Tag { get; }

        public void AddRef();
        public void RemoveRef();
        protected override void OnCreate();
        protected override JobHandle OnUpdate(JobHandle inputDeps);
        protected internal override void OnCreateForCompiler();
    }
}

I see it used in the example code.

    protected override void OnCreate()
    {
        base.OnCreate();
        m_GraphSystem  = World.GetOrCreateSystem<TAnimationGraph>();
        m_GraphSystem.AddRef();
        m_GraphSystem.Set.RendererModel  = NodeSet.RenderExecutionModel.Islands;
2

A comment from here: Unity.Animation.Samples/UnityAnimationHDRPExamples/Assets/Scenes/Introduction/Blendshapes/Blendshapes_ClipPlayer.cs at 23330ecc0ad2d6bbf7fcc220aac1df42299548c9 · Unity-Technologies/Unity.Animation.Samples · GitHub

// Increase the reference count on the graph system so it knows
// that we want to use it.