This is the best analogy I have heard in defense of formulaic prayer:
Imagine a young couple very much in love. The man wants very much to express his love for this wonderful woman. Being musically inclined, he grabs his guitar and heads to her house. Once there, he heads around to the backyard and throws a rock up at the window to catch her attention. She comes to the window and opens it wide as he starts to play the guitar and beautifully sing her favorite popular love song.
Then she screams at him, “Couldn’t you have made up the words as you went along!?”
Many people get hung up on pre-written prayers because they assume that they can only be used robotically. That is not the case at all. Rather (and this applies particularly to those prayers with such a rich and deep history), these prayers are like love songs that we use when our own words would almost get in the way. Of course we can and should pray spontaneously, just as we would have conversations with our beloved. But that doesn’t mean that formulaic prayers aren’t also a very important part of our spiritual arsenal.