This

My students often ask me, "Why do we need to pray if God already knows what we need?"  They miss the purpose of prayer.  Prayer is not trying to get God to change His will to fit your desires.  It's certainly not a way to fulfill your wants.  Neither do we pray to prove that God answers prayer--He answers every prayer His children pray to Him.  No, the purpose of prayer is to know God's will for your life and to embrace it--to surrender to the things He is bringing into your life to accomplish His purpose for your life--which is to transform you into His image so that you can be Christ-in-the-room wherever He sends you.  He as the Light of the World has passed on that responsibility to His disciples--His children.  So when I go to Him in prayer--and always be completely honest in presenting your requests to Him--and ask for physical needs or for circumstances to change or for whatever is on my heart, I can be sure that the "answer" He gives reveals exactly what I need to be transformed into His likeness.  If I don't get the physical things I pray for, it's because those physical things would be a detriment to my spiritual growth.  If I plead with Him to change my circumstances and He doesn't, it's because these trials are what I need to be conformed to His image--to become more like Him.  I have prayed weeping that God would change circumstances in my life, to allow anything to happen but "this one thing."  And His answer has been "this one thing" is what I'm going to use to make you more like Me so you can touch other lives with My presence.  He knows what I need.  I pray to understand that "this" is what I need, and then I must surrender to His will.  I must pray the prayer of My Savior.  "Father, let this cup pass from Me, but if not, nevertheless Thy will be done."  Yes, the purpose of prayer is to surrender to His will, to trust in His love, to trust in His wisdom, to deepen my commitment--my earnest desire--to be more like Him so that He can use me as a witness to His character--a living example of Him in the place He has planted me.
If I don't pray the prayer of surrender, I still must bear the cross.  But if I pray the prayer, nevertheless, Thy will be done,"  He changes me in three ways (at least).  Hebrews tells us that "for the joy that was set before Him, He endured the cross, despised the shame, and has sat down at the right hand of the Father."  When I surrender to His will in the "this," He empowers me with His joy--the joy that empowered Him to endure the cross.  I become the joy of Jesus to those around me.  And in the "this," His joy enables me to bear the "this" for His glory.  I become a reflector of His joy.  I live a joyous life in the midst of the trial.
He, also, gives me His peace.  "In everything by prayer and supplication, with thanksgiving, let your requests be made known to God.  And the peace of God--which is beyond human comprehension--will guard your heart and mind in Christ Jesus."  And the peace of God teaches me to be content in every situation.  And contentment teaches me that "I can do all things through Christ who is my strength."  The surrender in those prayers?  They are prayed "with thanksgiving"--with a heart that knows that He knows what I need and that no matter what happens my heart and mind will be fortified with His peace--God's peace, the peace that humans can't comprehend.  In the midst of the "this," I can continue to love right and think right because I have prayed, "nevertheless, Thy will be done."  I become a reflector of His peace to all those around me.
The prayer of surrender transforms me into His likeness in another way.  It empowers me to love with a holy love.  It enables me to be able to pray--even for those people who are responsible for my cross, "Father, forgive them for they don't know what they do."  By surrendering to His will, I become a reflector of God's holy love to those around me--a love that forgives, a love that is not diminished by whatever the response to my love might be.
The purpose of prayer is surrender to His perfect will--even to the "this" He has brought into my life.  And the answer to that prayer of surrender is the fullness of joy, the guarding power of God's peace in my heart and mind, and the empowerment to have a heart like His heart--to love those in my life with a holy love--a love eager to forgive even when forgiveness is not even asked for. 
Is the deepest desire of your heart to be like Him?  Then, present the desires of your heart to Him.  And then pray, in complete surrender, "not my will but Yours."  He will transform you into a disciple that radiates His joy, His peace, and His love as you bear your cross daily and follow Him.  "Oh, to be like Thee, blessed Redeemer."