There is no question that guiding the creation to the understanding and implications of Allaah's Oneness سبحانه و تعالى and Uniqueness (tawheed) is the overriding purpose of the Qur'aan. It is part of the nature of a human to want to know Allaah سبحانه و تعالى, his Lord and Creator. It is the Qur'aan that eloquently tells the human about his Lord such that he will then recognise the signs of his Lord, appreciate His actions and know His attributes. 
Through these teachings found in the Qur'aan, the human will increase his love and adoration for his Lord. He will worship Him with greater intensity and love and he will be a more devoted servant to Him. 
Through Allaah's سبحانه و تعالى clear description of His complete power over all things, His complete power to punish and His justice, the human learns to fear his Lord because of his own sins and shortcomings toward Him. At the same time, Allaah's سبحانه و تعالى clear description of His forgiveness and great mercy as well as the generous rewards He grants to the believers for their belief and good deeds increases the human's hope in Allaah سبحانه و تعالى. Hence, all of these aspects of the Qur'aan are intended to help the human know his Lord, have the proper relationship toward Hum and to have the feelings in his heart that make him act properly towards Allaah سبحانه و تعالى.
Thus when one reads the Qur'aan, he should realise that he is reading about his God and Lord. The Qur'aan is answering those basic human questions for him: Who is my Creator and God? What is my relationship to Him? Why did He create me? But, more importantly, Allaah سبحانه و تعالى is telling the human about Himself so the human may draw closer to Him and increase his adoration and worship Him.
That this is one of the overriding purposes of the Qur'aan can be seen in the following hadeeth wherein the Prophetصلى الله عليه وسلم identifies the greatest or most virtuous verse in the Qur'aan: Ubayy Ibn Ka'ab رضي الله عنه narrated that the Messenger of Allaah صلى الله عليه وسلم said to him, "O Abul-Mundhir! Do you know which verse of the Book of Allaah with you is the greatest?" He replied, "Allaah and His Messenger know best." Again the Prophet صلى الله عليه وسلم asked, "O Abul-Mundhir! Do you know which verse of the Book of Allaah with you is the greatest?" He replied, "I said, '[The verse,] Allaah, there is no god worthy of worship besided Him, the Ever-Living, the Ever-Sustaining'"? The Prophet صلى الله عليه وسلم then struck his chest and said, "By Allaah, may knowledge be made pleasant for you oh Abul-Mundhir [as you answered correctly]."