For if I pray in a tongue, my spirit prays 8. Public Tongues when interpreted are for the edification of the church. Once the two types of tongues are understood then it is clearer to see why the public use of tongues is a gift that not all believers use. It is one of the nine gifts and is motivated by the will of the Holy Spirit. However the private use of tongues is for personal edification.
Every believer needs to be built up.
Every believer needs to pray in the spirit. Classical Pentecostals believe that the ability to speak in personal tongues is not necessarily one of the nine gifts of the spirit but the initial evidence of being filled with the Holy Spirit. And He Himself gave some to be apostles, some prophets, some evangelists, and some pastors and teachers. One that is authorised and sent forth as a messenger to carry out an official duty on the sender's behalf. To establish churches.