Think about it like this:
GPU:
Oh, there’s a chair.
I see it uses the wooden material.
Let me get it.
Now lets draw this chair with this material.
Oh, there’s another chair.
I see it uses the metallic material.
Let me get it.
Now lets draw this chair with this material.
Oh, there’s a chair.
I see it uses the plastic material.
Let me get it.
Now lets draw this chair with this material!
Now imagine the chair has a fluffy pillow on it. You have to change the material twice for a single chair.