"Be anxious for nothing, but in everything by prayer and supplication, with thanksgiving, let your requests be made known to God; And the peace of God, which surpasses all understanding, will guard your hearts and minds through Christ Jesus." (vv.6-7)
Be anxious for nothing is a command, not an option. Paul wrote that everything (by prayer and supplication) are proper subjects of prayer. There are not some areas of our lives that are of no concern to God. These two aspects of prayer Paul speaks of here (prayer and supplication) are similar, but distinct.
Prayer is a broader word that can mean all of our communication with God, but supplication directly asks God to do something. Many of our prayers go unanswered because we do not ask God for anything. Here God invites us simply to let your requests be made known to Him. He wants to know.
Yes God already knows our requests before we pray them; yet He will often wait for our participation through prayer before granting that which we request. Adding thanksgiving to our prayers guards against a whining, complaining spirit before God when we let our requests be made known.
Notice than when we prayerfully do this His promise is that, "the peace of God, which surpasses all understanding, will guard your hearts and minds through Christ Jesus." We will be at rest in our minds, and relieved in our hearts that He is working all things out in His timing for us.
Therefore, let your requests be made known to God and enjoy His peace.
The Truth: "Peace I leave with you, My peace I give unto you: not as the world gives give I unto to you. Let not your heart be troubled, neither let it be afraid." (John 14:27)