The book of Luke, chapter 19, verses 1-10 tells the story of Zacchaeus, a rich tax collector whom Jesus visits. It's a brief event, simply the introduction and motivation for a longer parable that follows in the chapter, and Zacchaeus does not appear anywhere else in the Bible. Why then, do I and so many Lutherans - especially Sunday school-aged kids - know this story? Because somebody wrote a song about it! Everybody sing along: Zaccheus was a wee little man and a wee little man was he...
Aiding our memory is just one reason to set text to music. Martin Luther (along with plenty of other composers) understood that one effective way to share and teach theology is to set it to music. Of course, that was particularly important for people who lived in the early 16th century when so many congregants were illiterate or lacked access to the printed word. The words we sing in the hymns are meant to work in concert with the liturgy, scripture, sermon, and even the visual aspects of the church (liturgical color, for example) to present a cohesive worship service.
Thanks to the hymns of our tradition, Lutherans can boldly proclaim from memory: "A mighty fortress is our God" and "Lord, keep us steadfast in your word." Parts of the catechism show up directly in the hymnal, with Luther composing settings of the Lord's Prayer (ELW 746) and the 10 Commandments (to be sung throughout Lent at Bethany this spring). These hymns are lessons we carry with us in addition to being beautiful music.
The ancient Latin phrase lex orandi, lex credendi tells us that what we pray is what we believe. To give one reason for why we sing in church, we can add (in a rather Yoda-like formulation): lex orandi, lex cantandi, lex credendi, lex vivendi. What we pray, what we read, and what we sing, is what we believe and how we live.