One of the purposes in prayer is the unburdening of the soul. Sharing your pains and concerns with God. When you say to someone, I'll pray for you, its not to be an extra voice, or because your prayer will be heard better, its to simply say that your pain weighs heavily in my heart because I care about you, and because of that care and the pain, you will be a part of my prayers. Its a Christian way of saying I love you and your pains and worries mean a lot to me. On the humanistic side, knowing that people care about you has a very healing effect whether it be emotional, spiritual or physical.