What is appetite have to do with nutrition?

This questions bugged me a long time. Sometimes I really want to eat something, yet I am not hungry. Just a feeling in my throat that tells me to eat something specific, like a banana, apple, milk, or juice. And when I have one of these momentsd, I imagine (for example) milk running down my throat, and I can tell if its going to quench my desire just by imagining it. What does this all mean?