John 16:7-8 But I tell you the truth: It is for your good that I am going away. Unless I go away, the Counselor will not come to you; but if I go, I will send him to you. When he comes, he will convict the world of guilt in regard to sin and righteousness and judgment.
The Holy Spirit is a person. He is the third person of the Trinity: Father, Son and Holy Spirit. Being a person, he is a spirit. He is not any old run of the mill spirit. He is God the Spirit, The Holy Spirit. Being one with the Father and the Son, he takes from them and communicates the truth to us. Christ send him to those Christ chooses to send him. The process of receiving the Holy Spirit is determined by Jesus Christ. He sends Him to us.
The purpose or mission of the person of the Holy Spirit is clear. He will convict people of their lost state of life. He will stir up within them a longing that they can never seem to fill or meet. People will attempt to fill that longing with activity, knowledge, material things, etc. but will never find the hunger or thirst relieved. The Holy Spirit is unique in his role and the manner in which he does this. The lost will always grope around in the world looking or sensing a need. The do not even know will fill the need. It is futile situation. Not unlike the rat on a treadmill trying to get to the end of the wheel, and not understanding why it is taking so long.