It's an important question. I mean, isn't God all-knowing and stuff? Why do we need to ask Him for something when He not only knows what we need, but what we want and what is best for us?
There are so, so, so many ways that this can be answered, and many of them are very good ways to answer them. However, the majority of those answers tend to deal with the question in a way that, I believe, doesn't have the best focus. Essentially, oftentimes we look at prayer as something we do to, more or less, change God's Mind; or, at least change what God was going to allow have happen to us. But...what about us?
I mean, is God the One Who's Mind needs to be changed? Was God looking down upon creation, thinking everything was going hunky-dory, until He heard my prayer for a friend going through a very tough situation, and was completely shocked with this bit of information? Are we God's Little Tattle Tails?
I think we should look at prayer as not something that changes God in some way, but changes us in deep ways. When we pray for others, when we petition God to help those in need, we indicate that at least some part of us cares. It sanctifies us just a little bit more; and then just a lot bit more. Prayer is, in one way, a tool to perfect us, to make us want what God wants. To train our minds in a way that reflects God's Mind.
Now, let's apply that to the prayers we pray.
As a child, I was often told in church that recited prayers were "vain repetition" and thus evil. "Now I lay me down to sleep..." No, sir; none of that, here! So I had to come up with my own prayers: "Dear Jesus, thank You for this day, thank You for dying on the Cross for my sins, I pray for Grandpa and Papa to get saved (because they were Roman Catholics, so *obviously* they weren't "saved"--what?), and that/for X, Y, and Z, and in Jesus' Name, Amen." I kept repeating that formula with no problem...until I realized that I was saying, essentially, the same things, even with my "XYZ's".
Afraid that I might be having "vain repetition" in my prayers, I tried to change it up as much as possible. I ended up being very, very worried that I wasn't praying right because my last prayer wasn't different enough from the one before that.
In other words: I was trying to change my prayers rather than letting my prayers change me.
Today I have no worries for that. I pray recited prayers with absolutely no problem, especially every Sunday. I also pray prayers that are, well, conversational with God. After all, it is a relationship I have with The Messiah. But this relationship isn't a Buddy Christ relationship. One of us in the relationship is the unfaithful wife, the other the Faithful Husband. One of us is the broken and indebted son, the other the Rich and Loving Father. One of us needs to conform to the Other.
That's why I enjoy recited prayers: men and women more holy than I wrote them, and with them I can conform myself to the prayer, rather than the prayer conform to me. That's not to, once again, say that I DON'T pray freely; I still do. I need both types of prayer.
Think about the songs we sing to God in our services; are they not prayers? Do we not all say the same words? The Psalms was the prayerbook for both the Jews and the Early Church, and still is to this day for many within the Faith. Maybe songs and prayers are not very different.
Anyways, it's just a thought.