It is because he has to. He promised us in his word that whatever we ask believing he will do. Plus Jesus our mediator is always there reminding God about the blood of Calvary. And the peace of God, which transcends all understanding, will guard your hearts and your minds in Christ Jesus. It guards our hearts against the fiery darts of the devil and it brings us peace of mind releasing us from trouble.
Prayer is the start of accomplishment.
We will be unable to accomplish a task or construct without the proper supervision and the proper instructions. When we are perplexed about what to do or confused in our decision making, prayer can accomplish and complete the task. God will and does answer prayers. Matthew 6 gives us the answer. It is because sometimes we pray to be heard, or to hear ourselves pray.