Perhaps that the Samaritans will still get the same experience as those 120 people in Jerusalem: become filled with the Spirit by speaking in different tongues (Acts 2:4). That would be visual proof for the apostles that the Samaritans with their semi-heathen background are also completely included, through the same Spirit, in the church of Christ.
So, we read here that the apostles do not themselves have the power to control the Spirit and to hand it out themselves. Only God can give this gift, as he has promised. And the Spirit can only be received as gift from God by faith (see Acts 1:4, Acts 1:8; Acts 2:38–39).
15 who came down and prayed for them that they might receive the Holy Spirit,