Disclaimer: The commentary below was inspired by the Orthodox Study Bible.
When examining this scripture verse, here are some things that must be taken into account.
1. The Hypocrites miss the spirit of prayer, which is an intimate, personal communion with God that leads to the vision of His Glory. ( 1 Co 2:9).
2. Christ does not condemn the use of many words per se, but teaches that the words must express desire for communion with God. Therefore, it is vain repetition (pointless prayers without with the intention of having true communion with God) that are useless. Had repetition of prayer been condemned itself, Jesus would have not instructed us to repeat the Lord’s Prayer, nor would Luke 18:1 suggest to pray always; in addition to 1 Thess 5:17 demanding that we must “pray without ceasing.” In fact, the Book of psalms has many phrases in which we’re very repetitious! For example:
In Psalm 108 (109) King David continuously asks God for mercy. Verse 21: “Bless You, O Lord, O Lord, deal mercifully with me for your name’s sake, For Your Mercy is good.” He continues in verse 26: “Help me, O Lord my God; save me according to Your Mercy!” These are just a couple verses of the many examples of true repetitious prayer! (In fact, the continuous calling upon God’s mercy is exactly what we do when we pray the Jesus Prayer in the Eastern Churches).
3. True prayer is not telling God what He already knows and then telling Him what to do about it (a common practice we all unfortunately have done or continue to do), nor is it praying in-front of others to look pious. (I.E. worldly praise, the “reward” that the Pharisees got.) Rather, true prayer is (1) Humble. (I.E. “Go into your room” in Verse 6), (2) Personal. (“Pray to your Father”, Verse 6.) (3) Sincere. (“Do not use vain repetitions, Verse 7).