The Catechism of the Catholic Church (paragraph 524) says,
When the Church celebrates the liturgy of Advent each year, she makes present this ancient expectancy of the Messiah, for by sharing in the long preparation for the Savior's first coming, the faithful renew their ardent desire for his second coming. By celebrating the precursor's birth and martyrdom, the Church unites herself to his desire: "He must increase, but I must decrease." (Source:EWTM, Image: MorningOffering.com)