Paul writes,

What is the conclusion then? I will pray with the spirit, and I will also pray with the understanding.

1 Corinthians 14:15 - NKJV

While you're praying in tongues your mind should listen into your heart as the Holy Spirit intercedes. In this way you will start to become aware of how the Spirit is praying and direction comes to your own mind on how you should pray.

At this point pray with your understanding. You are to be inspired by the Holy Spirit's intercession in tongues to pray effectively in your own words and to continue to do so until your mind runs dry of Spirit-lead words. You move from praying in tongues to praying with the understanding and back again.

Why do we do this? Your prayer in tongues will indeed get things put right and flowing correctly but if you only pray in tongues you can miss out on some of the joy that comes from answered prayer. By following prayer in the spirit by prayer with your understanding you get to know exactly what you are petitioning for and you get to see just how wonderful the answer is.