The main purpose of the Holy Spirit is to testify that Jesus is the Christ the son of the living God. He will bring comfort and cheer to the sick and lonely. He will guide and direct your path to the way of truth and light. He is that feeling that you feel deep within you that something is right or wrong. He is the Lords messenger. Like God and Christ he cannot be in two places at the same time but his influence can be felt all over the world.
A person can have the Holy Spirit visit on occasion, but to have him permanently is what we should be aiming for.
He is the third member of the godhead, and is a convincing witness that the gospel of Jesus Christ is true.