Hope ya don't mind a lil' cut n' paste.
What was the purpose of tongues?
1) 1 Cor 14:22 says, “Tongues then ... are a sign for unbelievers”. The word “then” (“Wherefore” in
the KJV) means that this verse is referring back to the previous verse(s).
2) 1 Cor 14:21 says that God will speak to “this people”. The New Testament term “this people”
always refers to Jews and the unbelievers being referred to in 1 Cor 14:22 (from Isa 28) were Jews.
3) In Acts, whenever tongues was used Jews were present.
4) So, the purpose of tongues was to be a sign to unbelieving Jews. One sign was to show that salvation was for the Gentiles also (hence the foreign languages). Clearly, the sign is no longer needed.
5) Tongues was also a sign of judgment against the Jews. The quote in 1 Cor 14:21-22 comes from Isaiah 28:11-13 where judgment is pronounced against Israel. Their Temple was destroyed in 70AD.
Tongues is nowhere said to be a sign of a Christian being saved through receiving the Holy Spirit, nor is it said to be either a Prayer Language, the Baptism of the Holy Spirit or Tongues of Angels. These things can only be read into the text. The Bible only says that tongues was a sign to unbelieving Jews, nothing more.
The first church in Jerusalem was entirely Jewish and they badly needed convincing that Gentiles were part of God's salvation plan as they were fiercely, often violently, opposed to it. One example in Acts 11 shows this opposition and how tongues was used as a sign to verify that the Gentiles were in God's plan. Today, we have the Bible to explain the mystery of the church (Eph 3:1-6). Tongues, prophecy and knowledge are grouped because they had a limited purpose and when that purpose was served they were removed.