We must remember that the book of Acts is a transitional book and does not always illustrate what would later be the norm for the Church Age.

Below, are comments from The Bible Knowledge Commentary on the passage from Acts 8. I heartily agree with their assessment here and think you will find it helpful.

8:14-17. It was necessary for the apostles in Jerusalem to commission Peter and John to Samaria for several reasons. Normally the Holy Spirit baptizes, indwells, and seals at the moment of faith, but in this instance the delay served several purposes: (1) Peter and John’s prayer (for bestowing of the Holy Spirit) and their laying on of hands (resulting in the coming of the Spirit) confirmed Philip’s ministry among the Samaritans. This authenticated this new work to the Jerusalem apostles. (2) Also this confirmed Philip’s ministry to the Samaritans. This message Philip had preached was validated by the coming of the Spirit, a mark of the coming kingdom (cf. v. 12; Jer. 31:31-34; Ezek. 36:23-27; Joel 2:28-32). (3) Perhaps the most important aspect of God’s withholding the Spirit till apostolic representatives came from the Jerusalem church was to prevent schism. Because of the natural propensity of division between Jews and Samaritans it was essential for Peter and John to welcome the Samaritan believers officially into the church. The contrast between John’s attitude here and in Luke 9:52-54 is significant.