The disciples were interceding for Jerusalem with a oneness purpose. They wanted to see people come to believe in their resurrected Lord. When the Holy Spirit fell on those who were praying, I believe they were so filled with God's love that their prayers were set on fire for the people they knew. The first Christians prayed powerfully. Much of our contemporary prayer is either pointless or selfish.
Many congregational prayers are so general they are meaningless.
Most of our private prayers are wish lists of things we want God to give us or do for us. They prayed that God would enable them to preach with boldness and perform miraculous signs in the name of Jesus. After they prayed, the place where they were meeting was shaken. If we prayed as they prayed, we would be empowered as they were and we could preach as they preached. In his book, An Endless Line of Splendor, Professor Earle Cairns states that every great revival from the Great Awakening of 1726 to the present has begun with organized prayer groups.