The greater the charity of the Saints in their heavenly home, the more they intercede for those who are still on their journey and the more they can help them by their prayers; the more they are united with God, the more effective those prayers are. This is in accordance with Divine order, which makes higher things react upon lower things, like the brightness of the sun filling the atmosphere.
The world of pure spirits stretches between the divine nature and the world of human beings; because divine wisdom has ordained that the higher should look after the lower, angels execute the divine plan for human salvation: they are our guardians, who free us when hindered and help to bring us home.