In verse 8 Paul uses the use of a trumpet in warfare. If an unclear sound is given by the signalman then no one will know what to do or how to act. If what a person says cannot be understood he is only speaking in the wind. What he says is useless and profitless. There are many languages spoken in the world and all of them have a purpose.
All languages have the purpose of being able to communicate.
In fact, if we are just babbling we are as the barbarians. In the society of first century, anyone who did not speak the Greek language was considered a barbarian. The whole point is that the Corinthian practice of tongues edified no one, and even the true gift, if not interpreted helped no one. In that sense, the gift of prophecy was the greater gift because it lent to revelation, knowledge, and doctrine. The goal of all we do as a Church ought to be the edification of others.