If take question 'Do you want to eat?' as example, I don't understand meaning of 'do' because:  

* to do something in my understanding is to take some actions, to want
something is not an action, it is not something, that you 'do'.
* And in form of simple statement this question will be 'I want to eat',
not 'I do want to eat' (right?).

What meaning of 'do' in this question?