Paul was used by God to Receive and Deliver the Gospel of GraceThroughFaith. While the other Apostles were caught between Judaism and the Power of the Resurrection (Gal 2:16,) Paul was preaching that InChrist we are NewSpirit beings (2Co 5:17 Gal 6:15) who are to be Filled with the HolySpirit (Eph 5:18.)
The crux of the Gospel is Redemption, NewBirth and SpiritBaptism for the purpose of us being in intimate personal Relationship with God. We are to be One with Him (Joh 17:11 Joh 17:21 Joh 17:23.)
What has this got to do with Speaking in Tongues? To enter the SpiritRealm of Consciousness we must surrender our Soulish self to the NewSpirit being who is InChrist. In so doing, we communicate with God in the Spirit. That communication manifests as audible sounds coming from our mouth but not our Minds - it comes as the HolySpirit gives utterance.
The purpose of SpeakingInTongues is multi-fold but the most universal and personally powerful aspect is PrayerLanguage. This is what Paul was alluding to. He prayed in the HolySpirit and he prayed with the understanding also (1Co 14:15.)