A. First of all there are special slots throughout the prayer, even during the Amida, where we can (some say should) add our own prayer, in our own words.
Why does prayer need formalities? For structure. Maimonides writes: The Biblical Mitzvah of Prayer is that one should pray to G-d each day for his/her needs. Later the Rabbis developed a formal structure to be able to properly express that inner prayer.
Here’s an example: The purpose of an education is to gain knowledge, skills etc. So why bother with college? Go to a library, immerse yourself and forget about all this school structure. Ah, but people aren’t like that. We need structure, and operate best within a framework. Otherwise we aren’t as motivated, lack the knowledge to find more knowledge, don’t realize what they’re really reading etc.
Think of it as a lawyer coaching a (honest) client. The client knows his story best, but needs the lawyer to help articulate that personal conviction into legalese understood and accepted by the courts. I’ve heard Chassidic elders say: the prayerbook’s letters themselves are dumb and lifeless. Our heartfelt emotions give them life.
One more thing: Those who wrote the prayers (King David, The Great Assembly & others) knew what they were doing. To those who understand and appreciate them, these words are meaningful ’till this day. These words are “pregnant” with many rich layers of deep meaning hidden within.