Promises and vows
In many circumstances, the Christian is called to make promises to God. Baptism and Confirmation, Matrimony and Holy Orders always entail promises. Out of personal devotion, the Christian may also promise to God this action, that prayer, this alms-giving, that pilgrimage, and so forth. Fidelity to promises made to God is a sign of the respect owed to the divine majesty and of love for a faithful God.
Source: Catechism of the Catholic Church (Vatican.va)