Why were we created to serve if He is all-powerful? In Revelation 1:6


  and has made us to be a kingdom and priests to serve his God and
  Father--to him be glory and power for ever and ever! Amen.


From the bible we can clearly see that Angels serve God as messengers of information, warriors, destroyers, and protectors.

Why would all-powerful God need servants if he is all-powerful unless serving God means something else. And I'd like to know what the Catholic Church has to say about it?
The question is better answered by understanding why God created at all and CCC 293 tells us the God created all things not to increase his glory, but to show it forth and to communicate it, for God has no other reason for creating than his love and goodness.

The next point CCC 294 continues that God made us to be his sons through Jesus Christ, according to the purpose of his will, to the praise of his glorious grace, [...] The ultimate purpose of creation is that God who is the creator of all things may at last become all in all, thus simultaneously assuring his own glory and our beatitude.

Therefore for his glory and our beatitude, God made us to know Him, love Him and serve Him in this world, and be happy with Him forever in the next. [Cf. Penny Catechism 2].

But really, who is serving who when the LORD himself came not to be served but to serve, and to give his life as a ransom for many. 

What shall one render to the Lord for all his bounty to them? [Cf. Ps 116:12 (RSVCE)]

The way I look at it is that it is an exchange, just as lovers do, God has given us everything we have and in his love and mercy and goodness, even gives us the gifts for us to make a return to him.


  It is truly right and just, our duty and our salvation, always and
  everywhere to give you thanks, Lord, holy Father, almighty and eternal
  God. For, although you have no need of our praise, yet our
  thanksgiving is itself your gift, since our praises add nothing to
  your greatness, but profit us for salvation, through Christ our Lord.
  And so, in company with the choirs of Angels, we praise you, and with
  joy we proclaim: Holy, Holy, Holy Lord God of hosts . . . - COMMON
  PREFACE IV.