When we have prayed all the words we can think of, the Lord can pray through us in tongues as the Spirit of God speaks through us in languages we have not learned. The Lord can speak precisely what the person or situation needs and that only Jesus may know about.

"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. And he that searcheth the hearts knoweth what is the mind of the Spirit, because he maketh intercession for the saints according to the will of God." - Romans 8:26-27

"Praying always with all prayer and supplication in the Spirit, and watching thereunto with all perseverance and supplication for all saints;" - Ephesians 6:18