It's really not like telling your children what their purpose is, to love you...
In the 10 commandements, we are commanded to LOVE GOD WITH ALL OUR HEARTS.
I believe it is only by God's grace that one comes to God to serve, and when they do, they are giving a purpose in life.. To LOVE God will ALL the heart. That means that everything you do is filled with LOVE, ADORATION for God. Being born sinners, it is not always easy, even though it is exceptionally rewarding.
With a purpose to LOVE GOD WITH ALL THE HEART, many things are accomplished with acts of love, truth, charity, faith, and hope to make the world and the life within it, including themselves, better.