Wasn't the original purpose of tongues to make things more comprehensible to the listeners?
It's what Paul seems to suggest is the use of tongues when he writes to the church in Corinth. The idea is that if people come into church and hear you worshiping in a language they don't know, it means nothing, but if they hear you worshiping in their own language -- which you don't know -- they stand convicted that God is in the church (1 Corinthians 14:22). That certainly was the impact of the crowd who heard Peter's sermon early in Acts.
I still have no idea what the scriptural basis is for a "prayer language," though I know many Pentecostals and charismatics who swear by it.
Paul does say, "If I speak in the tongues of men and angels, but have not love, I am only a tinkling cymbal or resounding brass." But since that's the ONLY place in the entire Bible that Pentecostals have found to justify the notion of speaking in a heavenly prayer language, I consider that shaky reasoning. That would be like arguing that because the Torah refers to locusts as creatures that move "on all fours" that locusts really are four-legged and have two arms, or like saying that Job really did have skin on his teeth.
Every language has idiomatic expressions, and I'm inclined to consider "the tongues of men and angels" to be one of those until I hear a convincing argument otherwise. I'm also dubious because heretics like Montanus spoke in ecstatic tongues, as modern-day frauds like Robert Tilton do, and ecstatic utterances are found in other religionsm, from voodoo to Islam. That's by no means an unshakeable indictment of ecstatic utterances, but you have to admit that it's a stronger sign of God's presence if the utterance is in an authentic language unknown to the speaker, and not just random "Untie my bowtie" noises.
Remember, when the glossolalia first was manifest in the book of Acts, it had the express effect of Jews from all over the world hearing the gospel proclaimed in their own native languages -- even though the speakers were virtually all from Judea and Galilee.
Back when I was in the Assemblies of God, my pastor interpreted it as an indication that glossolalia was meant to be a normal experience, since every time the Holy Spirit came to a new group of people -- Jews, Samaritans and Gentiles -- it was manifest outwardly in the speaking of unknown tongues.