Why did God send the Holy Spirit to the earth after Jesus ascended?

The Holy Spirit has a threefold purpose for being on the earth.  He is here to reprove the lost of sin because they refuse to believe on Jesus as Lord and Savior (John 16:9).  By doing this they are blaspheming the Holy Spirit because He only points people to their need of Jesus as their sacrifice for sin (Mark 3:29; John 16:14).  The Holy Spirit also reproves the world of righteousness because Christ has ascended back to the Father (John 16:10).  The world is evil and wicked and because of this they are rebuked for a lack of righteousness.  The final purpose of the Holy Spirit is to reprove the world of judgment, because the world is already judged (John 16:11).  The world is already condemned and in need of a Savior and the Holy Spirit reveals this to all (John 3:17-18).