'For he who speaks in a tongue speaks not to men, but to God. For no man understands, but in the spirit he speaks mysteries. But he who prophesies speaks to men edification, and exhortation, and consolation.'
This is because the man who speaks with tongues, which has been their favourite test of spirituality, does not speak to men at all. He speaks to God. For no one understands him (note that Paul assumes that the tongues will not be understandable. (Unlike at Pentecost, that is not the point of these tongues). He may be speaking in the Spirit but he is speaking mysteries. It is of benefit to none but himself.
The gift of tongues is a gift by which men can speak in unknown languages to God. Paul describes speaking in tongues as 'speaking words' so that they would seem to be some form of language. But neither speaker nor hearer understand them. They are a means by which men speak to God, and as described here clearly contain an element of thanksgiving, although, unless the tongues are interpreted, only God knows about it. Yet their use brings private blessing to the heart. They provide some kind of spiritual relief and assistance in private worship whereby the heart is drawn to God. This is thus mainly a gift for private use and that is the question that Paul will deal with, for some of the Corinthians were making a great show of tongues in public.
Tongues which could actually be described as known languages have (rarely) been known in the present day, and have been evidenced, but it is not usual for them to be recognised, and it is not their purpose. And even so they did not have the purpose of edifying. The recognition of the language was usually purely 'accidental' because say a missionary was present who recognised the language. Pentecost was an exception. Sadly many who have enthusiastically sought to set them forth as real commonly known languages have in their ignorance often made fools of themselves. We need to beware of over enthusiasm not backed up by solid evidence.
But today so many are artificially worked up that it is doubtful whether they are genuine tongues at all, simply babbling. Whether that was so in Paul's time we do not know.
On the other hand the one who prophesies in love speaks to men, edifying, exhorting, consoling. Rather than him speaking to God for his own private blessing, God is speaking through him for the blessing of all. And the whole church is blessed. By 'edifying' is meant benefiting spiritually or improving morally, building up the inner man. Exhortation (parakaleo) encourages, and spurs on, and strengthens and comforts. Consolation comforts and nurtures and encourages. Prophecy of this type was not intended to produce new revelation.
Prophecy was an especially important gift in the early church because as the church spread it had to depend on only partly trained men. The special inspiration of men by the Holy Spirit was, next to the Scriptures and the Tradition of Jesus, the life-blood of the church. Today we are better trained. But we would do well to seek to prophesy by the Spirit as we preach.