An individual's spirit must first be recreated in the image and likeness of Christ (be born again). Then and only then will the Holy Ghost join Himself o that person's recreated spirit. When this joining takes place, it is evidenced by an experience known as glossilalia or speaking in tongues.
"And they were all filled with the Holy Spirit and began to speak with other tongues, as the Spirit gave them utterance." Acts 2:4
"While Peter was still speaking these words, the Holy Spirit fell upon all those who heard the word. And those of the circumcision who believed were astonished, as many as came with Peter, because the gift of the Holy Spirit had been poured out on the Gentiles also. For they heard them speak with tongues and magnify God...." Acts 10:44-46
"And when Paul had laid his hands on them, the Holy Spirit came upon them and they spoke with tongues and prophesied." Acts 19:6
Why is the joining of the Holy Ghost to a believer's recreated spirit evidenced by glossilalia or speaking in tongues?
Speaking in tongues evidences the joining of the Holy Ghost to a believer's recreated spirit because the believer's recreated spirit is in a sense spiritually comnustible or flamable. The believer's spirit can be ignited by the Holy Ghost (set on Holy Ghost fire). When this spiritual ignition takes place, it is evidenced by speaking in tongues.
The spiritual language of tongues flows out of the belly, bypassing the mind. Jesus said, out of the believer's belly would flow rivers of living water. (John 7:38) When the believer speaks in tongues, that individual is speaking from his/her own recreated spirit. The purpose of the language is direct communication with God.
Speaking in tongues is the doorway to the power of God. Jesus said the believer would receive power AFTER that the Holy Ghost is come upon him. Why AFTER? Because there is still some prayer work to be done.
When a believer speaks in tongues, that believer spiritually edifies (builds up) himself. "He who speaks in a tongue edifies himself, but he who prophesies edifiess the church." 1 Corinthians 14:4 Jude says that believers are to build themselves up by praying in the Holy Ghost. (Jude 20,21) Believers who pray in the Holy Ghost build themselves up and receive spiritual edification to reach the world for Jesus.