May the words of my mouth and meditation of my heart find favor before You (Psalms 19:15).
Why must we verbalize prayer? Since God knows our innermost thoughts, why don't we just meditate? Furthermore, why should we pray at all? Since God knows what is best for us, we should just trust that He will provide that which we need.
Let us consider the second question first. We do not pray in order to inform God of anything, for indeed He knows our needs better than we do. We pray in order to make ourselves aware of our dependence upon God. We are always at risk of deluding ourselves that we have control of our destinies. We may think that what we do and what transpires are indeed cause and effect. We therefore need to be reminded frequently that except where the principle of moral free choice applies, our destinies are controlled by God.
Why verbalize prayer? Speech alone characterizes us as humans and distinguishes us from lower forms of life. Animals undoubtedly think and feel, but only humans can speak. As we stand before God, we need to remember that we are human, and that as humans our goals and behavior should have the dignity of humanity. If we only pray for our physical needs and welfare, we have not advanced beyond the animal stage, and we are then what science calls Homo sapiens - intellectual animals, but animals nevertheless. We need to remember that we are much more than Homo sapiens, for we can aspire to spiritual achievements and goals.