It tells him what you do not understand, it shows how you think about things, and it gives the teacher some insight into how you learn. It also evokes the answer. Many time some of the best most enlightening discussions have come about from a question that drew out an answer that explained things in a way the teacher would never have used or thought of without the initial question. Questions usually come from those who are teachable. This is easily differentiated from good questions.
Good questions come from the truly teachable.
One needs to let go of the ego to be a good question asker. Such humility makes one a favorite student to any competent teacher. Humility is always endearing. Pride is definitely the cause of the un-teachable attitude. The best teacher will also have a humble teachable attitude.