The Baptism with the Holy Spirit as a separate event from conversion is a commandment in the book of Acts.
Which makes it anything but an optional thing.
Jesus commanded a number of things to His followers among which are the baptism in water, communion with bread and fruit of the vine in His remembrance, and baptism with the Holy Spirit.
Here is the command given to His disciples before His ascension:
“4 And while staying with them he ordered them not to depart from Jerusalem, but to wait for the promise of the Father, which, he said, “you heard from me; 5 for John baptized with water, but you will be baptized with the Holy Spirit not many days from now.”
These verses contain an emphatic statement: “he ordered them.” When the Creator of the universe gives an order there is no room for debate. The same order applies to Christians today, in addition to the other commandments that Jesus gave His disciples.
The reason for this commandment is obvious throughout the book of Acts as the Holy Spirit’s supernatural work is critical for effective evangelism, power over demonic forces, and sanctification of the believer.
One cannot be an effective witness for God without the filling with the Holy Spirit:
“But you will receive power when the Holy Spirit has come upon you, and you will be my witnesses in Jerusalem and in all Judea and Samaria, and to the end of the earth.” Acts 1:8
The born again believer must pursue the filling with the Holy Spirit, must earnestly desire the gifts of the Spirit, and must effectively use this empowerment for spreading the Gospel.