The bulk of the teaching on tongues in the Bible is not in Acts, but in 1 Corinthians. While there are a couple of instances in Acts that do mention people speaking in tongues, only Acts 2 mention that the words are understood by the bystanders. In 1 Cor 12-14, it is clearly understood that tongues require interpretation in order to be understood.
Researchers generally differentiate between
- glossolalia, speaking in unknown tongues
- xenoloalia, speaking in a know tongue, but in words not understood or learned by the speaker
The purpose of speaking in tongues is edification. Tongues + interpretation in public edifies the church and is comparable to prophecy in its effect. Tongues in private edifies the speaker.
Tongues are prayer and as such can be praise, thanksgiving and intercession. Being prayer of course also means that speaking in tongues is communion with God, practicing being in His presence.
The above can be understood from scripture. If we also listen to researchers the following can be said:
- While glossolalia exist in some non-christian settings, there is (AFAIK) no evidence of non ecstatic glossolalia in such settings. Older research (and Bible commentaries) tended to equate Christian glossolalia with ecstasy, trances, etc. (1 Cor 12-14 clearly indicates that speaking in tongues is a non ecstatic endeavor, but that does not mean that emotions may not run high, occasionally.)
- People who speak in tongues are not mentally unstable. In fact, having cross-referenced away other factors, research indicate that tongue-speaking people on average are more emotionally balanced and socially well adjusted than non tongue-speakers.
If we add testimonies from tongue-speakers about how they benefit from the practice, at least the following are common themes:
- Praying in tongues is a help when one does not know how to pray. Tongues means letting the Spirit produce the prayer that is optimal for the situation.
- Praying in tongues is associated with a release of the power of the Spirit to empower, heal or in any other way help. Quietly praying in tongues while laying hands on someone is a common practice, that combines these two points.
- Praying in tongues is a help in situations when ordinary words don't do justice to ones inner feelings of exuberance or grief.
- Praying in tongues is a help when one does not have the energy or concentration to pray ordinary prayers. When the activity of formulating words to pray are too strenuous, praying in tongues is usually effortless.
- Tongues function as a "gateway" gift, i.e. having been released to pray in tongues often is followed by other gifts, such as prophecy, gifts of healings, etc.
Saving the most controversial point for last: Some Pentecostals and charismatics see tongues as the initial evidence of the baptism in the Holy Spirit, which in turn is understood as a separate and non-repeatable experience from conversion. According to this view there is one Spirit-baptism, but possibly many "fillings". I personally do not share this view, since I think it makes a distinction between phrases where the Bible does not.
Additional sorce:The question answered by J. Rodman Williams, the first scholar to write a systematic theology from a renewal perspective.