Because of the People
John, chapter 11, beginning halfway through verse 41: "And Jesus lifted up His eyes and said, 'Father, I thank You that You have heard Me. And I know that You always hear Me, but because of the people who are standing by I said this, that they may believe that You sent Me.'"
It is true that scripture tells us that God knows what we need before we even ask Him. Knowing this, some might ask, "So why should we pray?"
Well firstly, it's a sign that we've humbled ourselves before God and we pray simply because He told us to. Secondly, our lives are supposed to be a demonstration to other people that they, too, can talk with God and can find the life that they have always longed for.
So the conclusion is, it's not just for our own sake, but for the people's sake that you and I would realize it's time again to pray.