Jesus told the disciples to wait in Jerusalem for the coming of the Holy Spirit because they would need Him in order to accomplish the work ahead of them. They needed His grace, His gifts, and His guidance in order to fulfill their mission. We also need the Holy Spirit to accomplish our mission. Just as we need the Lord to eliminate sin and attain heaven, we need the Lord to live out our daily life as Christians. He has not abandoned us (Mt 28:20). Our mission may not be identical to that of the original disciples (the Church is already established and most non-Christians today have encountered the story of Jesus in some way or another) but we very much still need the Holy Spirit to live out the life of Christ and to share the wonderful reality of salvation with others.