The posture we take when we pray depends upon how we relate to God during prayer. It can differ.
Kneeling would indicate that God is sovereign and the person praying expresses humility before God. When dedicating one’s life to God as in an ordination rite, or when surrendering the day’s burdens to God as in a bedtime prayer kneeling would be appropriate. In a worship service where there is a confession of sins, kneeling would be an appropriate posture then as well.
It is hard to be praying “Hallelujah” while kneeling. Then standing with arms raised to the skies seems a more appropriate posture. Praise is a form of prayer.
When walking the labyrinth seeking guidance, holding one’s hands with palms forward and slightly away from the body signifies one’s readiness to receive whatever is given. It suggests openness to the divine.
Sometimes, like in extreme joy or deep grief, the body will determines its posture spontaneously. If your feet want to dance with joy, let them. If your body wants to curl in a ball with grief, let it. The honest heart is more important than formal posture.
In some faith traditions kneeling, lying prostrate or somewhere in between these positions is taught as conforming to expectations. There is something positive in these traditions. When kneeling is a habit, it helps shape the spiritual approach to God. When any action such as kneeling is done in a larger community it binds people together spiritually and psychologically. Mostly it is done to reinforce the transcendence of God and the humility of human beings.
Personally, I would encourage anyone to try different postures and be aware of how they affect you. Then pray in whatever way is sincere and meaningful as the occasion requires. What you are supposed to do is pray with an honest and open heart.