Truth and wisdom require that the heavens have inhabitants appropriate to them as the earth has. According to the Shari’a, those various beings are called angels and spirit beings. Reality requires it to be thus, for despite its small size and insignificance, the earth being filled with living and conscious beings, then emptied from time to time and once again repopulated suggests, indeed makes it clear, that the heavens too, in which are magnificent constellations and are like adorned palaces, should be filled with conscious and percipient creatures. Like men and jinn, those creatures are spectators of the palace of the universe, the observers of the book of creation, and the heralds of the sovereignty of dominicality. For the universe is arrayed and embellished with innumerable adornments, decorations, and ornaments, and self-evidently requires the thoughtful gazes of those who will appreciate it and wonder at it. Certainly, beauty requires a lover and sustenance is given to the hungry. However, man and jinn are able to perform only a millionth of this endless duty, this grand viewing, this extensive worship. That is to say, endless sorts of angels and spirit beings are necessary to perform these endless duties and diverse worship. 

For further read:

15th Word, http://erisale.com/index.jsp?locale=en#content.en.201.191

29th Word, 1st Aim  http://erisale.com/index.jsp?locale=en#content.en.201.521