Not sure why some place their hands in a palms-together position, unless it's simply a sign of devotion or contemplation. At my church, most people just fold their hands in their laps. Some people raise their hands while praying, as Moses did when he sought God's attention to the concerns of the Israelites.
Jews pray standing, seated, swaying, hitting their breast with their closed fist, etc. Mormons pray with their arms folded. Children pray with their hand steepled and adults with their heads bowed or kneeling. The adoption of a posture during prayer is to show our internal state, or to show respect to the Almighty.