Since the mind gives us so much trouble, what is its real purpose?
The mind is the "pivot" of the senses. When untrained, the mind directs the body to fulfill whatever desires the senses have, good or bad. Intelligence is meant to control the mind, and operates on two levels—spiritual and material—offering arguments why other needs are more important than fulfilling sense desires. For example:
Senses: "I want to party."
The mind: "How can I arrange my life so that I can party?"
Material intelligence: "No, no! A party now is not good! You have an exam. Better to study now."
Spiritual intelligence: "You don't really want to party. You want enjoyment, but this type of enjoyment is temporary and will ultimately lead to misery. Go to the kirtan instead! Then you will really be happy!"
As we advance in Krishna consciousness, the mind becomes controlled by the intelligence and ultimately by the soul, our true self. The self is connected to the spiritual intelligence; they sit together like two birds in a tree. Part of our job as human beings is strengthening the intelligence so it serves the soul's desire. This will help us achieve the goal of life.
Keep in mind also that "atma" is defined in different ways in different places; sometimes it means the body, sometimes the mind, but most often it means the soul within the body. When the senses, mind or material intelligence dominate our consciousness, then we—the jiva (soul)—try to exploit and enjoy the material energy. When our mind is guided by spiritual intelligence, then our real sense of self—the realization that the atma is our true identity—pervades our consciousness. Then we can be fully happy and absorbed in Krishna Consciousness.