“And when you pray, you must not be like the hypocrites. For they love to stand and pray in the synagogues and at the street corners, that they may be seen by others. Truly, I say to you, they have received their reward.” Matthew 6:5

Jesus says we are to avoid the sin of hypocrisy in prayer because it is a misuse of the purpose of prayer - diverting it from the glory of God to the glory of ourselves. And we are not to go on and on in prayer - the Greek “battalogeo” meant "to stammer", some thought it referred to Battus, King of Cyrene who stuttered, others to Battus the poet who wrote really long winded poems. Either way we are not to "heap up empty phrases" or use "meaningless repetition" when we pray. Prayers that are all words with no meaning have no place in our lives - mechanical mindless words don't fly with God.