It usually gives us a feeling about its subject. She does not mean that a truck can actually sing. Rather, she is speaking figuratively. She is referring to road noises as music. By using the word song, and suggesting music, she brings joyful feelings to mind.
Poets use figurative language almost as frequently as literal language.
Otherwise, a poem may make no sense at all. Of course, the poet is not using literal language. He doesn't mean that the wallpaper literally jumped off the walls. Rather, he is using figurative language. This exaggeration suggests the power of the storyteller.