Watching young children writing out Latin vocab words this fall in my class made this response by Fr. David Moser to a question on prayer all the more poignant.
"Is it ok to pray spontaneously or should I always use formal prayer? Is it ok to just 'pour my heart out'...or just know that God knows what is going on?"
It is certainly a good thing to pray spontaneously. The prayer book is like a "primer" for prayer - a lesson book on how to pray. The scripture says that "we
do not know how to pray as we ought" and so the Holy Spirit teaches us to pray. How does the Holy Spirit teach us - in the school of prayer that is the Church.
Those formal prayers in the prayer book are the examples of how to pray, they are the "pouring out of the heart" of people who were experienced in prayer (the saints). We begin to learn to pray by mimicking the examples.
When you learned to write in school, weren't you give letters to trace over and over until you could do them without thinking, and then words to trace over and over and so on. Even now you use those same letters and words in your writing - the letters and words you traced have now become your own and are the means of expressing your own innermost thoughts and feelings.
We "trace over the lines" of the prayers by copying them over and over until they sink in and become "natural", then we use those prayers as the letters and words of our own innermost spiritual expressions. That's the "role" of the "formal" prayers in the prayerbook.
The Fathers, when speaking about prayer of this kind, also teach us that it is a good thing to add our own spontaneous prayers in as much as we are able to our private prayers. Once you learn the basics of how to pray from the prayer book, then you combine and re-combine the elements of the prayers that you learned into new prayers - your own prayers.