One of the numerous benefits of praying in other tongues is the fact that it gets you synchronized with the will, purpose, and timing of God. Those who haven’t understood this awesome benefit of speaking in tongues would rather use vain repetitions; numerous words in their understanding to try to express themselves to God. The Lord Jesus already pointed out the fruitlessness of praying that way: "But when ye pray, use not vain repetitions, as the heathen do: for they think that they shall be heard for their much speaking" (Matthew 6:7).
What do you want to tell a God who knows everything? He gave us the ability to pray in other tongues. When you pray in tongues, your spirit is stirred to know the mind of God. In 1 Corinthians 14:15, the Apostle Paul said, "...I will pray with the spirit, and I will pray with the understanding also...." He gave pre-eminence to praying in other tongues because that way, the Spirit stirs your heart and guides you to pray the will of the Father.
Sometimes, some Christians unwittingly ask God for things that could ruin them. That wouldn’t be the case if they had prayed fervently in other tongues: they’d have been guided by the Spirit as to what to ask for, and how! The Spirit helps our limitations in prayer; He bears us up in our weaknesses. He grants us the right spiritual expressions in our communion with the Father: "Likewise the Spirit also helpeth our infirmities: for we know not what we should pray for as we ought: but the Spirit itself maketh intercession for us with groanings which cannot be uttered" (Romans 8:26).