Petition I think is the main part of prayer, and it makes all the other parts of prayer work. For example, we can confess our sins, but sins won’t be forgiven without asking God to do it. Also, we can praise and thank God, but without the promise of petition, praise and thanks would not be genuine and real; for praise and thanksgiving has its basis in the presence of the Holy Spirit in us, of which we cannot obtain except through petition. That is, we received the Holy Spirit and the filling of Him each day through our asking; and it is this very Spirit that brings us to praise and causes us to be thankful. Therefore, by petition confession is answered, praise is joyous, and thanksgiving is genuine and heartfelt.
The petition part of prayer has at least the following three purposes:
1. That we may receive the things we need. A higher and overall purpose for prayer is that God’s will would be done on earth as it is in heaven (Matt. 6:10). But a big part of His will is that our daily needs be met. The last three requests of The Lord’s Prayer (Matt. 6:11-13) outline for us what these are: (1) Our daily, physical provisions; (2) daily forgiveness; and (3) daily guidance and protection. All the things that we will ever need in our life I think are outlined in these three points, and it is God’s purpose for prayer that we receive them. It is the way God has planned it, the way that He has designed prayer.
2. To show our dependence on Him. Another purpose of petition I think is to express our dependence on God, that we cannot survive without Him, that we are totally dependent on Him for daily provisions, daily pardon, and daily protection.
3. To bring glory to the Father. Jesus said to His disciples just before He ascended to His Father, “And whatever you ask in My name, that will I do, that the Father may be glorified in the Son” (Jn. 14:13). Jesus speaks these words to us too. And they give to us the greatest purpose for prayer. Just think, by our petitions we can have the great and wonderful experience of bringing our Father glory. Yes, when we pray for anything in His name and receive it, He will be glorified in the Son.
Prayer A to Z 
God is all knowing, all powerful, Almighty, the creator of all. His plan is irrevocable. Every event under heaven is planned & has fore knowledge. Our prayers of petition will not chg that projectile. Christians must accept God’s plan for their lives. I assume you agree that God owns the blueprint.
Our prayer have great value, There are many times in Scripture where prayer has changed God’s mind. Read the example of when Hezekiah was told by the prophet that he was going to die, But Hezekiah prayed for his life and God added fifteen years to his life (2 Kings 20:1-7). Oh yes, we should always pray. God wants us always to pray, Prayer changes things. Prayer changes how God works, otherwise why would He want us to pray.