In fact Bhagwat Gita is the "Song of the Divine" told by the Divine (God himself) to His disciple Arjuna. In it God (Lord Krishna) himself teaches Arjuna the philosophy of truth. In it Lord Krishna tells Arjuna about the philosophy of rebirth or reincarnation & how individual soul transmigrates from one body to the other. It also teaches you the difference between a Soul & Supersoul (God). It also tells you, what is Karma & how you can do the Karma effectively through Karmayoga.
In Bhagwat Gita, God teaches basically the four paths through which one can understand the philosophy of soul or oneself. First one is "Sankhya yoga" or the "Yoga of Knowledge". In it one can know the God through knowledge or through one's intellect. The second one is "Karma yoga, or the Yoga of Action." In it one can know the God through offering all of his / her actions to God. Third one is "Dhyana yoga" or the "The Yoga of Meditation". In it one can know the God through Meditation of God. Fourth one is "Bhakti Yoga" or the "The Yoga of Devotion." In it one can know the God through having devotion, love towards God.
So if you want to have a stress free life & the peace of mind then you should read Bhagwat Gita. If you want to know the mystery of Life & Death, rebirth etc. then also you should read this great "Divine Song". It is basically the essence of all the Vedas, Puranas, Upanishads & all other Hindu scriptures. If one is able to understand the divine philosophy of Bhagwat Gita then one is able to understand all the great scriptures as it is the essence of all these great books.
Otherwise, reading Bhagwat Gita gives peace of mind, clears all doubts about life, makes one understand people better & let's one come closer to God.
Here below are thoughts of century's greatest saint Mahatma Gandhi about Bhagwat Gita & why he used to read this great philosophy of Bhagwat Gita.
"When doubts haunt me, when disappointments stare me in the face, and I see not one ray of hope on the horizon, I turn to Bhagavad-Gita and find a verse to comfort me; and I immediately begin to smile in the midst of overwhelming sorrow. Those who meditate on the Gita will derive fresh joy and new meanings from it every day." - Mahatma Gandhi
Jai Sri Krishna.