Once you know the answer to the classic spiritual question "How many angels can dance on the head of a pin?", the next question many would ask is "Why would an angel dance on the head of a pin?".  The simple answer is: why not?  A pin isn't any better or any worse a place for an angel to dance than any other.  Dancing on the head of a pin or the head of a king is all the same to an angel.  To an angel, everything is spirit and matter is just spirit that is a bit sick and run down so it doesn't make any difference.  Some saints and holy persons experience this, St. Francis is a primary example.  He referred to everyone and everything as his brothers and sisters because he could see the spirit hidden under the mask of matter.
    The next question one might ask would probably be "Why should angels dance at all?".  The answer to that is also simple: Joy.  Joy at knowing truth, joy for knowing God and joy at being a servant of God and part of his great plan.
    We also can be part of that plan.  The choice is ours, serve God intentionally or serve Satan by default.