Parshas Vayishlach begins with the account of Jacob's preparation for his meeting with his estranged brother, Esau. Jacob was very concerned about this meeting, and the Torah states that he turned to God in prayer. The Zohar (1:169a) tells us that Jacob's prayer serves as a model for all mankind of how to approach God in prayer.
One of the lessons that the Zohar derives from the prayer of Jacob comes from the verse (32:12), "Rescue me, please, from the hand of my brother, from the hand of Esau, for I fear him, lest he come and strike me, mother and children." The Zohar says that this teaches us that, in prayer, a person should fully state his concerns. Although, in principle, Jacob could have simply said, "Rescue me, please!", Jacob went on to state, in detail, who he was afraid of - "my brother", "Esau" - and what he was afraid Esau would do to him. The Zohar says that this should be a model for our prayers as well.
The question that remains, of course, is why should this be so? God certainly knows what we need, and He certainly knows what our intent is. Why is it necessary to lay it all out in such detail? In fact, this raises the even more basic question, why do we need to pray - verbally - at all?
These questions clearly demonstrate that, in its essence, the function of prayer is not to inform God of our needs - which would be an absurdity - but to accomplish some important change in the person who is praying. As many commentaries point out, the Hebrew verb for prayer, l'hispallel, is in the reflexive form. The object of the verb is the one who is praying!
In his Guide for the Perplexed (3:36), Maimonides explains that the function of prayer is "to firmly establish the true knowledge" that God is concerned about us and is able to help us, and that nothing happens by chance. Prayer reinforces in our mind the knowledge that God is the source of all that we have and, through this, prayer enables us to reconnect to God and become worthy of receiving new blessings from Him.
This is why a short, vague, prayer is insufficient. By clearly stating our concerns, we remind ourselves that God is fully involved in our lives, not just in some general way, but in every detail. Our father, Jacob, by speaking to God in detail about his concerns about meeting his brother, was reinforcing in his own mind his knowledge of God's detailed providence over his life. We too, in our prayers, whether in our formal prayers from the Siddur, or in our informal personal prayers, should use these opportunities to come closer to God by internalizing the basic message of all prayer, that God cares about us and is only waiting for us to open ourselves up to His blessings.