What do those who practice the gift of 'tongues' consider the purpose to be?

In Acts 2, we read of how the apostles received the gift of tongues so that people heard the gospel in their own languages. Is the gift of tongues, then, specifically evangelistic or does it have a purpose in a congregation where there are only believers? I was under the impression that only Charismatics currently practice this gift. What do they consider the purpose to be and are there others who actually practice the gift that consider the purpose to be different? How are these purposes defended using either Old or New Testaments?